an embodiment of the present invention will hereinafter be described by the drawings . a drag roller control apparatus according to this embodiment is equipped in a web type electronic printing machine that performs predetermined printing on a web being fed . this web type electronic printing machine , as shown in fig1 serially feeds a web 6 unwound from a rolled web 5 set in a web feeding unit 4 to printing units 7 a ˜ 7 d via a paper feeding roller ( drag roller ) 1 , an intermediate roller ( drag roller ) 2 , and a paper discharging roller ( drag roller ) 3 , and performs predetermined printing on the obverse of the fed web 6 . note that this web type electronic printing machine is identical in construction and operation with the aforementioned prior art ( see fig5 ) and therefore a detailed description thereof is omitted . also , in fig1 the same reference numerals will be applied to the same parts as the conventional web type electronic printing machine shown in fig5 . now , a description will be made of the drag roller control apparatus according to this embodiment . the drag roller control apparatus of this embodiment , as shown in fig1 is constituted by a temperature monitoring unit includes thermometer 8 for checking the exterior circumferential surface temperature ( outside periphery temperature ) of the intermediate drag roller 2 equipped in a web type electronic printing machine such as the aforementioned , a controlling unit 10 for issuing a control signal for controlling the rotational speed of the intermediate drag roller 2 so as to adjust a circumferential speed of the intermediate drag roller 2 , which speed corresponds to the exterior circumferential surface temperature checked by the temperature monitoring unit 8 , to a preset speed corresponding to a reference exterior circumferential surface temperature ( reference temperature ) of the intermediate drag roller 2 , and a rotational speed changing unit 9 for changing the rotational speed of the intermediate drag roller 2 in accordance with a signal from the controlling unit 10 . note that the preset speed is a target speed corresponding to the preset reference exterior circumferential surface temperature under which a web can be moved without any shift of a printing position . the thermometer 8 here is , for example , a non - contact type thermometer such as a radiation thermometer , a quartz thermometer , a fiber - optic thermometer and the like . this thermometer 8 is disposed at a position adjacent to the intermediate drag roller 2 so as to be opposed to the exterior circumferential surface of the intermediate drag roller 2 in order to directly measure the exterior circumferential surface temperature of the intermediate drag roller 2 . thus , by setting the thermometer 8 at a position such that the exterior circumferential surface temperature of the intermediate drag roller 2 can be directly measured , a variation in the diameter of the intermediate drag roller 2 can be calculated more accurately . in this embodiment , while the thermometer 8 is provided at a position such that the exterior circumferential surface temperature of the intermediate drag roller 2 can be directly measured , the present invention is not limited to this . for instance , as shown in fig2 the thermometer 8 may be provided at a position such that the exterior circumferential surface temperature of the intermediate drag roller 2 can be indirectly measured via the web 6 pulled by the intermediate drag roller 2 . in this case , there is a need to make a predetermined correction of the measured value of the thermometer 8 , but there is an advantage that the flexibility of a design ( manufacturing ) increases with respect to a space for installation . also , the thermometer 8 is not to be limited to a non - contact thermometer like this embodiment , but may be of another type . for instance , it may be a contact - type thermometer that contacts the main body or shaft end portion of the intermediate drag roller 2 to measure the exterior circumferential surface temperature . as an example of this contact - type thermometer , there is a thermistor , a thermocouple , a resistor bulb , an optical thermometer , etc . the rotational speed changing unit 9 is equipped with an ac servo motor 11 adapted to be connected to the shaft end portion of the intermediate drag roller 2 through a belt 11 a , and a motor controller 9 a for controlling the rotational speed of the ac servo motor 11 on the basis of a signal from the controlling unit 10 . the signal from the controlling unit 10 is sent to the motor controller 9 a , which in turn controls the rotational speed of the ac servo motor 11 . in this manner , the ac servo motor 11 is rotated and driven . with this , the rotational speed of the intermediate drag roller 2 is changed . note that the rotational speed changing unit 9 also serves as a drive unit , because it drives the intermediate drag roller 2 to change the rotational speed of the intermediate drag roller 2 . although the rotational speed changing unit 9 in this embodiment is equipped with the ac servo motor 11 , the rotational speed changing unit 9 is not to be limited to this , but may be various units or means . for instance , the ac servo motor 11 , a continuously variable transmission , and a continuously variable transmission controller may be provided as the rotational speed changing unit 9 . in this case , the main shaft of the ac servo motor 11 is connected to the shaft end portion of the intermediate drag roller 2 through the continuously variable transmission . based on a signal from the controlling unit 10 , the continuously variable transmission controller controls the gear ratio of the continuously variable transmission to suitably control the rotational speed of the ac servo motor 11 . the controlled rotational speed is transmitted to the intermediate roller 2 . with this , the rotational speed of the intermediate roller 2 is changed . also , although the rotational speed changing unit 9 in this embodiment is equipped with the motor controller 9 a or the continuously variable transmission controller , they may be included in the controlling unit 10 to be described later . the controlling unit 10 is constructed so that it feeds back the measured value regarding the exterior circumferential surface temperature of the intermediate drag roller 2 measured by the thermometer 8 , thereby setting the rotational speed of the intermediate drag roller 2 so that the circumferential speed of the intermediate drag roller 2 reaches a circumferential speed corresponding to the exterior circumferential surface temperature of the intermediate drag roller 2 . a signal for setting the rotational speed of the intermediate drag roller 2 is output from the controlling unit 10 to the aforementioned rotational speed changing unit 9 . for this reason , the controlling unit 10 is constructed so as to have a function ( rotational speed setting means ) of setting the rotational speed of the intermediate drag roller 2 so that the circumferential speed of the intermediate drag roller 2 reaches a target circumferential speed , based on the exterior circumferential surface temperature of the intermediate drag roller 2 . in this embodiment , the controlling unit 10 sets the rotational speed n of the intermediate drag roller 2 in accordance with the exterior circumferential surface temperature of the intermediate drag roller 2 by a temperature - speed conversion table 10 a , thereby setting the rotational speed of the intermediate drag roller 2 . note that the controlling unit 10 may be constructed so that it sets the diameter of the intermediate drag roller 2 in accordance with the exterior circumferential surface temperature of the intermediate drag roller 2 by a temperature - diameter conversion table 10 b and then sets the rotational speed of the intermediate drag roller 2 in accordance with the diameter of the intermediate drag roller 2 by a diameter - speed conversion table 10 c . with this , the rotational speed n of the intermediate drag roller 2 is calculated . here , there is a relation that the circumferential speed of the intermediate drag roller 2 will vary , if the exterior circumferential surface temperature or diameter of the intermediate drag roller 2 varies . therefore , the exterior circumferential surface temperature or diameter of the intermediate drag roller 2 is referred to as a parameter that has influence on the circumferential speed of the intermediate drag roller 2 . in addition , the controlling unit 10 may be constructed so as to have a function ( calculation means ) of calculating a variation in the diameter of the intermediate drag roller 2 from a variation in the exterior circumferential surface temperature of the intermediate drag roller 2 . with this , the rotational speed of the intermediate drag roller 2 is set in accordance with the calculated variation in the diameter so that the circumferential speed of the intermediate drag roller 2 reaches a target circumferential speed . next , for the drag roller control method ( control algorithm ) that is carried out by the drag roller control apparatus according to this embodiment , a description will be made in reference to fig3 . in the drag roller control method of this embodiment , if the operation of the web type electronic printing machine is started ( step s 10 ), the exterior circumferential surface temperature of the intermediate drag roller 2 is measured by the thermometer 8 ( step s 20 ). note that step s 20 is referred to as a temperature checking step . the exterior circumferential surface temperature ( temperature value ) measured by the thermometer 8 is sent to the controlling unit 10 . the controlling unit 10 sets the rotational speed n of the intermediate drag roller 2 in accordance with the exterior circumferential surface temperature measured with the thermometer 8 by the temperature - speed conversion table 10 a , in which prospective rotational speeds corresponding to various exterior circumferential surface temperatures of the drag roller are previously set , for converting exterior circumferential surface temperature to rotational speed n . with this , the rotational speed n of the intermediate drag roller 2 is set ( step s 30 ). note that step s 30 is referred to as a temperature / speed setting step . in this embodiment , while the rotational speed n of the intermediate drag roller 2 is calculated by the temperature - speed conversion table 10 a , the present invention is not limited to this . as shown in fig4 the controlling unit 10 may be equipped with a temperature - diameter conversion table 10 b , in which prospective drag roller diameters corresponding to various exterior circumferential surface temperatures of the drag roller are previously registered , for converting exterior circumferential surface temperature to the diameter of the intermediate drag roller 2 and a diameter - speed conversion table 10 c , in which prospective rotational speeds corresponding to said drag roller diameters registered in said temperature - diameter table are previously set , for converting the diameter of the intermediate drag roller 2 to the rotational speed n . with the temperature - diameter conversion table 10 b , the diameter of the intermediate drag roller 2 is first set in accordance with the exterior circumferential surface temperature measured by the thermometer 8 ( temperature / diameter setting step ). then , with the diameter - speed conversion table 10 c , the rotational speed n of the intermediate drag roller 2 is set in accordance with the diameter of the intermediate drag roller 2 set by the temperature - diameter conversion table 10 b ( diameter / speed setting step ) with this , the rotational speed n of the intermediate drag roller 2 is set . subsequently , the set rotational speed n is issued to the rotational speed changing unit 9 of the intermediate drag roller 2 as a rotational speed changing signal ( step s 40 ). note that step s 40 is referred to as a control signal issuing step . then , the rotational speed changing signal is input to the motor controller 9 a of the rotational speed changing unit 9 . the motor controller 9 a operates an ac servo motor 11 , thereby changing the rotational speed of the intermediate drag roller 2 . based on the exterior circumferential surface temperature measured by the thermometer 8 , the circumferential speed of the intermediate drag roller 2 is controlled ( step s 50 ). note that step s 50 is referred to as a rotational speed changing step . in the above - mentioned rotational speed setting step , while the rotational speed n of the intermediate drag roller 2 is calculated with the conversion tables 10 a , 10 b , and 10 c by rotational speed setting means ( controlling unit 10 ), the present invention is not limited to this . for example , as shown in fig3 assuming that the diameter of the intermediate drag roller 2 in the initial state , i . e ., state at the time of start of operation at normal exterior circumferential surface temperature ( reference temperature ) is d 0 and the rotational speed in this case ( basic rotational speed ) is n 0 and also assuming that the diameter of the intermediate drag roller 2 after expansion ( at the time of elevated temperature ) is d ( d 0 & lt ; d ) and the rotational speed corresponding to this diameter d ( after - expansion rotational speed ) is n ( n 0 & gt ; n ), there is a need to control the rotational speed of the intermediate drag roller 2 in view of the following equation ( 6 ) and the state of the web 6 fed by the web feeding unit 4 in order to make the circumferential speed ( target circumferential speed ) v of the intermediate drag roller 2 constant . that is , the intermediate drag roller 2 rise in temperature and expands , so that the diameter becomes the diameter d after expansion which is greater than the diameter d 0 at the time of start of operation . for this reason , there is a need to set the rotational speed of the intermediate drag roller 2 to the after - expansion rotational speed n slower than the basic rotational speed n 0 , as a basis . in other words , in the above - mentioned rotational speed setting step ( step s 30 ), the controlling unit 10 may be constructed so as to perform the calculation step of calculating the diameter d after expansion of the intermediate drag roller 2 on the basis of the exterior circumferential surface temperature measured by the thermometer 8 and also perform the set step of setting the rotational speed of the intermediate drag roller 2 to the rotational speed n slower than a previously set rotational speed n 0 in the initial state so that the circumferential speed of the intermediate drag roller 2 reaches a predetermined target circumferential speed , based on the relation between the previously set diameter d 0 and rotational speed n 0 of the intermediate drag roller 2 in the initial state , i . e ., state at the time of start of operation at normal exterior circumferential surface temperature ( reference temperature ) and the diameter d after expansion calculated in the aforementioned manner . with this , the web transfer quantity at the time of start of operation and the web transfer quantity after expansion can be made equal to each other , as indicated by the following equation ( 7 ): note that the variation in the diameter ( d − d 0 ) and the rate of change in the diameter ( d / d 0 ) of the intermediate drag roller 2 can be calculated based on various conditions , such as a coefficient of volume expansion , which are determined by the material of the intermediate drag roller 2 , a difference in temperature , roller dimensions and like . therefore , as described above , in the case where the rotational speed of the intermediate drag roller 2 is set based on the diameter of the intermediate drag roller 2 , the variation in the diameter ( d − d 0 ) and the rate of change in the diameter ( d / d 0 ) of the intermediate drag roller 2 are calculated . then , based on these , the rotational speed of the intermediate drag roller 2 can be set . the drag roller control apparatus according to this embodiment can stably transfer the web 6 to the printing units 7 a ˜ 7 d regardless of a variation in diameter due to the exterior circumferential surface temperature change of the intermediate drag roller 2 and can remove the disadvantage that the printing position shifts due to a state variation such as the tension or elongation of the web 6 , because it has the aforementioned construction and functions . with this , there is an advantage that printing quantity can be considerably enhanced . in connection with this , the number of sheets of damaged paper resulting from defective printing is reduced , so that there is also an advantage that cost reduction can be achieved . next , a description will be made of modifications of the drag roller control apparatus and the drag roller control method according to this embodiment . in the aforementioned embodiment , the exterior circumferential surface temperature of the intermediate drag roller 2 is measured by the thermometer 8 . then , based on the measured temperature , the diameter of the intermediate drag roller 2 is calculated . in accordance with this diameter , the rotational speed of the intermediate drag roller 2 is set so that the circumferential speed of the intermediate drag roller 2 reaches a target circumferential speed . on the other hand , in this modification , the diameter of the intermediate drag roller 2 is directly measured by a laser ray , a high frequency , or the other measuring means ( diameter monitoring unit ). in accordance with this diameter , the rotational speed of the intermediate drag roller 2 is set so that the circumferential speed of the intermediate drag roller 2 reaches a target circumferential speed . note that the remaining construction is the same as the above - mentioned embodiment . in this case , the controlling unit 10 is constructed so that it sets the rotational speed of the intermediate drag roller 2 so that the circumferential speed of the intermediate drag roller 2 reaches a target circumferential speed , based on the diameter of the intermediate drag roller 2 measured by the diameter measuring means . the controlling unit 10 here sets the rotational speed of the intermediate drag roller 2 in accordance with the diameter of the intermediate drag roller 2 measured with the diameter measuring means by the diameter - speed conversion table 10 c , thereby setting the rotational speed of the intermediate drag roller 2 . here , there is a relation that the circumferential speed of the intermediate drag roller 2 will vary , if the diameter of the intermediate drag roller 2 varies . therefore , the diameter of the intermediate drag roller 2 is referred to as a parameter that has influence on the circumferential speed of the intermediate drag roller 2 . note that , as with the aforementioned embodiment , the rotational speed of the intermediate drag roller 2 may be set in accordance with the variation or rate of change in the diameter of the intermediate drag roller 2 calculated based on the diameter of the intermediate drag roller 2 directly measured . the drag control method , which is carried out by the drag roller control apparatus according to the modification constructed as described above , is as follows : in this drag roller control method , if the operation of the web type electronic printing machine is started , the diameter of the intermediate drag roller 2 is directly measured by the diameter measuring means ( diameter measuring step ). the diameter measured by the diameter measuring means is sent to the controlling unit 10 . the controlling unit 10 sets the rotational speed n of the intermediate drag roller 2 in accordance with the diameter detected with the diameter measuring means by the diameter - speed conversion table 10 c for converting diameter to rotational speed n ( diameter / speed setting step ). with this , the rotational speed n of the intermediate drag roller 2 is set ( rotational seep setting step ). subsequently , the set rotational speed n is transmitted to the rotational speed changing unit 9 of the intermediate drag roller 2 as a rotational speed changing signal . then , the ac servo motor 11 constituting the rotational speed changing unit 9 is operated , whereby the rotational speed of the intermediate drag roller 2 is changed . with this , the circumferential speed of the intermediate drag roller 2 is controlled based on the diameter measured by the diameter measuring means ( rotational speed changing step ). with this , as with the aforementioned embodiment , the web 6 can be stably transferred regardless of a variation in the diameter of the intermediate drag roller 2 . with this , the modification of the aforementioned embodiment can remove the disadvantage that the printing position shifts due to the tension or elongation variation of the web 6 . with this , there is an advantage that printing quantity can be considerably enhanced . in connection with this , the number of sheets of damaged paper resulting from defective printing is reduced , so that there is also an advantage that cost reduction can be achieved . in addition , in this embodiment , while the drag roller control apparatus is provided for controlling the intermediate drag roller 2 , the drag roller control apparatuses of the same construction may be provided for controlling the paper feeding roller 1 and the paper discharging roller 3 . in this case , the traveling state ( transferring state ) of the web 6 can be controlled more stably . in the above case , the thermometer 8 as the temperature measuring means needs to be provided at a position adjacent to the intermediate drag roller 2 . also , the thermometers 8 need to be provided at positions adjacent to the paper feeding roller 1 and the paper discharging roller 3 . furthermore , for the paper feeding roller 1 and the paper discharging roller 3 , the same control as the drag roller control apparatus regarding the intermediate drag roller 2 needs to be performed . with this , the traveling state ( feeding state ) of the web 6 with respect to the printing units 7 a ˜ 7 d can be controlled more ideally . moreover , the thermometer 8 as the temperature measuring means may be provided only at a position adjacent to the intermediate drag roller 2 . based on the measured information , a variation in the diameter of the intermediate drag roller 2 is calculated and variations in the diameters of the paper feeding roller 1 and the paper discharging roller 3 are also calculated . for the paper feeding roller 1 and the paper discharging roller 3 , the same control as the drag roller control apparatus regarding the intermediate drag roller 2 is performed . with this , although the structure is simpler , the traveling state ( feeding state ) of the web 6 with respect to the printing units 7 a ˜ 7 d can be made better . while the present invention has been described with reference to the preferred embodiment thereof , the invention is not to be limited to the details given herein , but may be modified within the scope of the appended claims .