in the following , embodiments of the present invention will be described with reference to the accompanying drawings . fig3 is a diagram showing the construction of a first embodiment of a shift register controlling circuit according to the present invention . the shift register controlling circuit of fig3 includes a shift register ( s / r ) 40 , a right / left - array selecting circuit 41 , a shift control signal generating circuit 42 , and a shift control signal generating circuit 43 . the shift control signal generating circuits 42 and 43 generate shift control signals clk_l and clk_r for provision to the word decoder sets 14 - 1 and 14 - 2 , respectively . the word decoder sets 14 - 1 and 14 - 2 are as illustrated in fig1 and fig2 . the shift control signal clk_l is supplied as the shift control signal cntl shown in fig2 to the word decoder set 14 - 1 corresponding to the left - side cell array unit 13 - 1 . the shift control signal clk_r is supplied as the shift control signal cntl shown in fig2 to the word decoder set 14 - 2 corresponding to the right - side cell array unit 13 - 2 . in fig3 , the shift register 40 is provided separately from the word line selecting shift registers for selecting a word line selecting decoder at the time of refresh operations ( e . g ., the word line selecting shift registers 31 shown in fig2 ), and is provided for the monitoring purpose to indicate which one of the word line selecting shift registers is in the selected state . based on the output of the shift register 40 , the right / left - array selecting circuit 41 generates signals reflz and refrz indicative of which one of the left and right cell array units is in the selected state . in response to the signals reflz and refrz , the shift control signal generating circuits 42 and 43 generate the shift control signal clk_l to be supplied to the left - side word decoder set 14 - 1 and the shift control signal clk_r to be supplied to the right - side word decoder set 14 - 2 , respectively . fig4 is a diagram showing an example of the circuit construction of the shift register 40 . the shift register 40 of fig4 includes 2n shift registers ( s / r ) 50 - 1 through 50 - 2 n . the shift registers 50 - 1 through 50 - 2 n receive a clock signal clk , and make a “ 1 ” bit propagate to a next shift register in synchronization with the clock signal clk . the clock signal clk is a pulse signal for requesting a shift at the time of refresh operations . namely , one cycle of the clock signal clk corresponds to one cycle of refresh requests . the output of the first shift register 50 - 1 is denoted as r 2 , the output of the n - th shift register 50 - n denoted as 11 , the output of the n + 1 - th shift register 50 - n + 1 denoted as 12 , and the output of the 2n - th shift register 50 - 2 n denoted as r 1 . these signals r 1 , r 2 , 11 , and 12 are supplied to the right / left - array selecting circuit 41 . fig5 is a diagram showing an example of the circuit construction of the right / left - array selecting circuit 41 . the right / left - array selecting circuit 41 includes nor gates 51 through 56 and inverters 57 through 62 . when the “ 1 ” bit is stored in the n − 1 - th shift register 50 - n − 1 , for example , the signal reflz indicating the left - side word decoder set 14 - 1 is high , and the signal refrz indicating the right - side word decoder set 14 - 2 is low . as the “ 1 ” bit propagates to the n - th shift register 50 - n , the signal 11 becomes high . in response , the output of the flip - flop comprised of the nor gates 53 and 54 changes from high to low , resulting the signal refrz indicative of the right side being changed from low to high . at the next timing , the “ 1 ” bit propagates to the n + 1 - th shift register 50 - n + 1 , which causes the signal 12 to become high . in response , the output of the flip - flop comprised of the nor gates 51 and 52 changes from low to high , resulting in the signal reflz indicative of the left side being changed from high to low . when a transition is made from the left - side word decoder set 14 - 1 to the right - side word decoder set 14 - 2 as described above , the signal refrz indicative of the right - hand side changes from low to high first , and , then , the signal reflz indicative of the left - hand side changes from high to low at the next clock cycle . by the same token , when a transition is made from the right - side word decoder set 14 - 2 to the left - side word decoder set 14 - 1 , the signal reflz indicative of the left - hand side changes from low to high first , and , then , the signal refrz indicative of the right - hand side changes from high to low at the next clock cycle . fig6 is a diagram showing an example of the circuit construction of the shift control signal generating circuit 42 . the shift control signal generating circuit 42 of fig6 includes an nand gate 71 and an inverter 72 . as illustrated , the shift control signal generating circuit 42 is a simple and circuit , and outputs the clock signal clk as the shift control signal clk_l only when the signal reflz indicative of the left - hand side is high . the shift control signal generating circuit 43 has the same construction as that shown in fig6 , and outputs the clock signal clk as the shift control signal clk_r only when the signal refrz indicative of the right - hand side is high . as a result , the left - side shift control signal clk_l is supplied as a clock signal only when the left - side word decoder set 14 - 1 is selected , and the right - side shift control signal clk_r is supplied as a clock signal only when the right - side word decoder set 14 - 2 is selected . accordingly , the control signal cntl is selectively supplied to only one of the word decoder sets 14 - 1 and 14 - 2 , thereby avoiding needless current consumption in the unselected decoder set . as was described in connection with the right / left - array selecting circuit 41 of fig5 , the signal reflz indicative of the left - hand side and the signal refrz indicative of the right - hand side overlap with each other for one clock cycle at the time of a transition when the selected position is shifted between the left - hand side and the right - hand side . namely , both of these signals become high at the same time for one clock cycle . accordingly , the left - side shift control signal clk_l and the right - side shift control signal clk_r each generate one clock pulse simultaneously at the time of transition . this ensures that the shift data “ 1 ” of the word line selecting shift registers is handed over between the left - hand side and the right - hand side without a problem . the word line selecting shift registers 31 provided in the word decoder set 14 - 1 and the word decoder set 14 - 2 may be configured into a hierarchical structure in which they are divided into a plurality of blocks . fig7 is a drawing showing word line selecting shift registers and word decoders organized in a hierarchical structure . fig7 shows word decoders 81 , word line selecting shift registers ( s / r ) 82 , refresh control signal generating circuits 83 , and refresh block latches 84 . the word decoders 81 are grouped into a plurality of blocks , and each of these blocks is provided with one refresh control signal generating circuit 83 and one refresh block latch 84 . the refresh control signal generating circuits 83 corresponding to the left - hand - side word decoder line receive the shift control signal clk_l , and the refresh control signal generating circuits 83 corresponding to the right - hand - side word decoder line receive the shift control signal clk_r . each refresh block latch 84 receives and holds “ 1 ” from the last word line selecting shift register 82 of the preceding block , thereby indicating that the corresponding block is a block for selection . while the corresponding block is a block for selection , the output of the refresh block latch 84 is “ 1 ”, which is supplied to the refresh control signal generating circuit 83 . the refresh control signal generating circuit 83 generates shift control signals six and siz ( i : an integer indicating a block ) based on the shift control signal clk_l ( or clk_r ) while the output of the refresh block latch 84 is “ 1 ” ( i . e ., while the corresponding block is being selected ). in synchronization with these shift control signals , the “ 1 ” data is successively shifted through a series of shift registers comprised of the word line selecting shift registers 82 . fig8 is a timing chart showing the operation of the word line selecting shift registers having the hierarchical structure shown in fig7 . as shown in fig8 , only one of the shift control signals clk_l and clk_r ( see fig3 ) generated by the shift register 40 , right / left - array selecting circuit 41 , and shift control signal generating circuits 42 and 43 based on the clock signal clk is in the activated state , corresponding to which one of the right and left arrays is selected . further , the shift control signals clk_l and clk_r are generated such that one clock pulse overlaps at the time of transition . a signal rbi ( i : an integer indicating a block ) generated by the corresponding refresh block latch 84 becomes high when the corresponding block is selected . as shown in fig8 , the shift control signal s 3 z ( and s 3 x ) supplied to the word line selecting shift registers 82 of the third block is activated during the period in which the signal rb 3 is high ( i . e ., when the third block is in the selected state ). further , the shift control signal s 4 z ( and s 4 x ) supplied to the word line selecting shift registers 82 of the fourth block is activated during the period in which the signal rb 4 is high ( i . e ., when the fourth block is in the selected state ). as shown in fig8 , the shift control signal of a given block ( e . g ., s 3 z ) and the shift control signal of the immediately following block ( e . g ., s 4 z ) are generated such that one clock pulse overlap at the time of transition . in this manner , provision is made to generate one overlapping clock pulse not only at the time of a transition between the right and the left but also at the time of a transition between the blocks . the signals r 1 , r 2 , 11 , and 12 shown near the bottom of fig8 are supplied from the shift register 40 to the right / left - array selecting circuit 41 ( see fig4 and fig5 ). fig9 is a diagram showing an example of the circuit construction of the word line selecting shift register 82 ( or word line selecting shift register 31 ). the word line selecting shift register 82 of fig9 includes pmos transistors 91 through 97 , nmos transistors 98 through 104 , and transfer gates 105 and 106 . the transfer gates are each comprised of a pmos transistor and an nmos transistor connected in parallel . the pmos transistors 92 and 93 and the nmos transistors 101 and 102 constitute a first latch . further , the pmos transistors 96 and 97 and the nmos transistors 103 and 104 constitute a second latch . in response to the shift control signal six and siz ( i : an integer indicative of a block ) supplied from the refresh control signal generating circuit 83 , the transfer gates 105 and 106 are opened or closed . with the transfer gate 105 being open , the first latch stores input data “ in ”. as the transfer gate 105 closes and the transfer gate 106 opens , the data of the first latch is transferred to the second latch for storage therein . the data stored in the second latch will be retained therein until the transfer gate 106 , subsequently closing , opens again at the next cycle . in this manner , a register that retains data for one clock cycle is provided . fig1 is a drawing showing an example of the circuit construction of the word decoder 81 ( or the word decoder 30 ). the word decoder of fig1 includes nmos transistors 111 through 121 and pmos transistors 122 through 125 . at the time of refresh operation , the selecting signal sel is set equal to low . as a result , the nmos transistor 114 is tuned off , and the nmos transistor 116 is turned on . a terminal a receives an output of the word line selecting shift register . as the word line selecting shift register selects the word decoder of interest , the terminal a becomes high , resulting in the nmos transistor 115 being conductive . in response , a node b is changed to low , which causes the main word line mwl to be placed in the selected state ( low ). if access is requested from an exterior of the device while refresh operations are successively performed for each word line , the selecting signal sel is set equal to high . in this case , the main word line mwl corresponding to a specified address is placed in the selected state in response to the address signal supplied from the exterior . in the case of fig1 , the node b becomes low to put the main word line mwl in the selected state ( low ) when all address signals add - a through add - c are high . fig1 is a drawing showing the construction of a second embodiment of the shift register controlling circuit according to the present invention . in fig1 , the same elements as those of fig3 are referred to by the same numerals , and a description thereof will be omitted . in the construction shown in fig1 , the shift register 40 of the construction shown in fig3 is replaced by a counter -& amp ;- decoder circuit 40 a . the counter -& amp ;- decoder circuit 40 a includes a counter for counting up ( or counting down ) in synchronization with the clock signal clk , and further includes a decoder for decoding the count of the counter . with this provision , it is possible to provide the function equivalent to that provided by the shift register 40 . in should be noted that as decoder outputs , only the counter decoded values corresponding to the signals r 1 , r 2 , 11 , and 12 of fig4 may be output . this makes it possible to implement a decoder by use of a small - scale circuit . fig1 is a drawing showing the construction of a third embodiment of the shift register controlling circuit according to the present invention . in fig1 , the same elements as those of fig3 are referred to by the same numerals , and a description thereof will be omitted . in the construction shown in fig1 , a check as to which one of the right and left arrays is in the selected state is made by utilizing the outputs of the word line selecting shift registers 82 arranged in the word decoder sets 14 - 1 and 14 - 2 , rather than using the shift register 40 as in the first embodiment or the counter -& amp ;- decoder circuit 40 a as in the second embodiment . specifically , the output of the word line selecting shift register 82 situated at the turning - back point from the left - hand side to the right - hand side is denoted as r 1 , and the output of the word line selecting shift register 82 situated at the turning - back point from the right - hand side to the left - hand side is denoted as 11 . further , the output of the word line selecting shift register 82 situated immediately following the turning - back point from the left - hand side to the right - hand side is denoted as r 2 , and the output of the word line selecting shift register 82 situated immediately following the turning - back point from the right - hand side to the left - hand side is denoted as 12 . to be specific , in fig7 , for example , an output po 2 on of a word line selecting shift register 82 is r 1 , and an output po 300 of a word line selecting shift register 82 is r 2 . with this provision , signals r 1 , r 2 , 11 , and 12 equivalent to those of fig4 are obtained . the signals r 1 , r 2 , 11 , and 12 extracted from the word decoder sets 14 - 1 and 14 - 2 as described above are supplied to the right / left - array selecting circuit 41 . the operations of the right / left - array selecting circuit 41 and the shift control signal generating circuits 42 and 43 are the same as those of the first embodiment previously described . it may be necessary to transfer the signals r 1 and r 2 through long - distance wires . to this end , buffers 131 through 134 are provided . fig1 is a drawing showing the construction of a fourth embodiment of the shift register controlling circuit according to the present invention . in fig1 , the same elements as those of fig3 are referred to by the same numerals , and a description thereof will be omitted . in the third embodiment shown in fig1 , the outputs of the word line selecting shift registers 82 situated immediately following the turning - back points are extracted for use . namely , in fig7 , for example , the output po 300 of the word line selecting shift register 82 situated immediately following the turning - back point needs to be extracted . in such configuration , there is a need to extract signals from the word line selecting shift registers densely arranged in the word decoder sets , and it is difficult to secure sufficient space for wires for extracting signals . in the fourth embodiment shown in fig1 , provision is made to extract an output signal from a word line selecting shift register situated at the last stage of the word decoder block situated immediately following a turning - back point , rather than extracting a signal from a word line selecting shift register situated immediately following the turning - back point . namely , in fig7 , for example , what is to be extracted is not the output po 300 of the word line selecting shift register 82 situated immediately following the turning - back point , but an output po 3 on of a word line selecting shift register 82 at the last stage of the word decoder block situated immediately following the turning - back point . since there is sufficient space between word decoder blocks , such configuration as described here makes it possible to easily secure sufficient space for wires . in the first through third embodiments , the output signals reflz and refrz of the right / left - array selecting circuit 41 overlap each other for one clock cycle at the time of transition between the right - hand side and the left - hand side . in the fourth embodiment , however , the output signals reflz and refrz of the right / left - array selecting circuit 41 end up overlapping for one block at the time of transition between the right - hand side and the left - hand side . namely , if the number of the word line selecting shift registers 82 in one block is k , these signals overlap each other for a duration of k clock cycles . this slightly degrades the effect of current reduction . if division into blocks is finely made , however , such degradation is negligible . fig1 is a drawing showing the construction of a fifth embodiment of the shift register controlling circuit according to the present invention . in fig1 , the same elements as those of fig3 are referred to by the same numerals , and a description thereof will be omitted . in the fifth embodiment , signals extracted from the word line selecting shift registers include only the signals r 1 and 11 at the turning - back point . in addition , a right / left - array selecting circuit 41 a is provided in place of the right / left - array selecting circuit 41 used in the first through fourth embodiments , and receives the signals r 1 and 11 . further , shift control signal generating circuits 42 a and 43 a are provided in place of the shift control signal generating circuits 42 and 43 used in the first through fourth embodiments . fig1 is a drawing showing an example of the circuit construction of the right / left - array selecting circuit 41 a . as shown in fig1 , the right / left - array selecting circuit 41 a includes nor gates 141 and 142 and inverters 143 and 144 . each time the signal r 1 or 11 indicating a turning back becomes high , the flip - flop comprised of the nor gates 141 and 142 is inverted as to its state . as a result , the output signals reflz and refrz serve to indicate which one of the right and left word decoder sets is in the selected sate . fig1 is a circuit diagram showing the circuit construction of the shift control signal generating circuit 42 a . the shift control signal generating circuit 43 a has the same circuit construction . the shift control signal generating circuit 42 a of fig1 includes inverters 151 through 157 , gated inverters 158 and 159 , transfer gates 160 and 161 , a nor gate 162 , and an and gate 163 . the inverter 154 and the gated inverter 158 together form a first latch . the inverter 155 and the gated inverter 159 constitute a second latch . when the signal reflz is high , an node n that is an inverse of the output of the nor gate 162 is fixed to high , so that the input clock signal clk is output as the shift control signal clk_l without any change . the signal reflz changes to low thereafter . since the output of the second latch remains high despite this change , the node n also remains high . then , the change to low of the signal reflz propagates through the first latch and the second latch , resulting in the node n changing to low one clock cycle later . with the node n changing to low , the shift control signal clk_l is placed in an inactive state ( fixed to low ). as described above , the shift control signal generating circuits 42 a and 43 a have the period extending function to extend the duration of shift control signal generation by one clock cycle of refresh requests , providing an extension after the switching of selected states between the right - hand side and the left - hand side . fig1 is a timing chart showing the operation of the word line selecting shift registers having a hierarchical structure shown in fig7 with respect to the fifth embodiment . among the signals shown in fig1 , signals refrz , reflz , n ( r ), and n ( l ) illustrated near the bottom of the figure differ from those shown in fig8 . the signals refrz and reflz serve to indicate which one of the right and left word decoder sets is in the selected state , as was described in connection with fig1 . the signals n ( l ) and n ( r ) are the signal of the node n of the shift control signal generating circuit 42 a ( see fig1 ) and the signal of the node n of the shift control signal generating circuit 43 a , respectively . as was described in connection with fig1 , the signals n ( l ) and n ( r ) are extended by one cycle compared to the signals reflz and refrz , respectively . because of this , the shift control signals clk_l and clk_r overlap each other by one clock cycle at the time of switching between the right - hand side and the left - hand side . fig1 is a drawing showing the construction of a sixth embodiment of the shift register controlling circuit according to the present invention . in fig1 , the same elements as those of fig3 are referred to by the same numerals , and a description thereof will be omitted . the sixth embodiment relates to a configuration in which a small - scale shift register circuit is used as a monitor - purpose shift register circuit . in the sixth embodiment shown in fig1 , a shift register ( s / r ) 40 b , counters 171 - 1 through 171 - n , and a signal selecting circuit 172 are provided in place of the 2n - bit shift register 40 shown in fig3 . the shift register 40 b is an n / n - bit shift register ( i . e ., a shift register circuit comprised of n / n stages ) where n is the number of the word line selecting shift registers provided in one line . further , there are a total of n counters 171 - 1 through 171 - n . the counters 171 - 1 through 171 - n count up by one each time data “ 1 ” shifts all the way to the last stage ( n / n - th stage ) in the monitoring - purpose shift register 40 b . from the monitoring - purpose shift register 40 b , the first stage output and the n - th state output are extracted and supplied to the signal selecting circuit 172 as po 00 and po - n . further , the carry - up signals of the respective counters 171 - 1 through 171 - n are also supplied to the signal selecting circuit 172 as flag 1 , flag 2 , . . . , and flagn . the signal selecting circuit 172 performs a logic operation on these supplied signals to generate the signals r 1 , r 2 , 11 , and 12 . fig1 is a drawing showing the construction of a seventh embodiment of the shift register controlling circuit according to the present invention . in fig1 , the same elements as those of fig1 are referred to by the same numerals , and a description thereof will be omitted . in fig1 , the shift register 40 b is replaced by a counter -& amp ;- decoder circuit 40 c . the counter -& amp ;- decoder circuit 40 c includes a counter for counting up ( or counting down ) in synchronization with the clock signal clk , and further includes a decoder for decoding the count of the counter . with this provision , it is possible to provide the function equivalent to that provided by the shift register 40 b . in should be noted that as decoder outputs , only the counter decoded values corresponding to the signals r 1 , r 2 , 11 , and 12 may be output . this makes it possible to implement a decoder by use of a small - scale circuit . fig2 is a circuit diagram showing an example of the circuit construction of the signal selecting circuit 172 . the signal selecting circuit 172 of fig2 is directed to an example in which n is 2 , and includes and gates 181 through 184 , nand gates 185 through 188 , and inverters 189 and 190 . an output f 1 ( shown as flag 1 in fig1 and fig1 ) of the first stage counter circuit 171 - 1 and an output f 2 ( shown as flag 2 in fig1 and fig1 ) of the second stage counter circuit 171 - 2 are decoded by the and gates 181 through 184 and the inverters 189 and 190 , thereby determining which lap is being performed by the shift register 40 b or 40 c . based on this determination , either the first - stage output po 00 or the n - th - stage output po - n is selectively output . this successfully generates the signals r 1 , r 2 , 11 , and 12 as signals indicating the occurrence of switching between the word line selecting shift register lines and also as signals indicating advancement of one stage after such switching . although the present invention has been described with reference to embodiments , the present invention is not limited to these embodiments . various variations and modifications may be made without departing from the scope of the claimed invention . for example , the above embodiments have been described with reference to an example in which the word decoder sets are arranged in two lines . even if three or more lines are provided , shift control signals may be generated in the same manner as in the disclosed embodiments so as to deactivate the shift control signals in the unselected lines , thereby reducing needless power consumption .