hereinafter , an image forming apparatus according to an embodiment of the invention will be described with reference to the drawings . fig1 shows a diagram schematically illustrating a configuration of an image forming apparatus ( mfp , multi - functional peripheral ) to which the invention is applicable . the image forming apparatus 10 shown in fig1 includes an image forming apparatus body 11 , a sheet feeding unit 12 , and an image acquisition unit 13 . the image forming apparatus body 11 outputs image information , for example , as an output image called as hard - copy or printout . the sheet feeding unit 12 can supply a recording sheet of an arbitrary size used in the image output with respect to the image forming apparatus body 11 . the image acquisition unit 13 acquires the image information , which is subjected to image formation in the image forming apparatus body 11 , as image data from an object ( document ) holding the image information . the image acquisition unit 13 includes a document table ( document glass ) 13 a supporting the object and an image sensor , for example , a ccd sensor , converting the image information into the image data . an illumination device ( not shown ) radiates illumination light to the document set on the document table 13 a , and the image acquisition unit 13 converts the light reflected from the document into an image signal by the ccd sensor . an instruction input unit , that is , a controller panel ( operation unit ) is provided at a predetermined position ( not shown in fig1 ) in the image forming apparatus 10 , which instructs the image forming apparatus body 11 on a start of image formation or the image acquisition unit 13 on a start of reading the image information of the document . the image forming apparatus body 11 includes : a first to a fourth photoconductive drums 14 a to 14 d holding a latent image ; developing units 15 a to 15 d developing the latent image by supplying developers , that is , toners to the latent image held on the photoconductive drums 14 a to 14 d ; a transfer belt 16 sequentially holding toner - images held on the photoconductive drums 14 a to 14 d ; cleaners 17 a to 17 d removing toners remaining on the photoconductive drums 14 a to 14 d from the photoconductive drums 14 a to 14 d , respectively ; a transfer unit 18 transferring the toner image held on the transfer belt 16 to a sheet - shaped recording medium , such as a plain paper or an ohp sheet formed of a transparent resin sheet ; a fixing unit 19 fixing the toner image transferred to the recording medium by the transfer unit 18 to the recording medium ; and an exposure unit 20 forming the latent image on the photoconductive drums 14 a to 14 d . the image signals obtained from the image acquisition unit 13 are sent to the exposure unit 20 . the photoconductive drums 14 a to 14 d rotate in arrow directions , respectively , and the surfaces thereof are uniformly charged in a charging unit ( not shown ) before being exposed by the exposure unit 20 . light images corresponding to y ( yellow ), m ( magenta ), c ( cyan ) and bk ( black ) are radiated to the photoconductive drums 14 a to 14 d , so that the latent images corresponding to the y , m , c and bk are formed . the first to fourth developing units 15 a to 15 d contain an arbitrary color of toners of y ( yellow ), m ( magenta ), c ( cyan ) and bk ( black ) used for obtaining a color image and develop the latent images held on each of the photoconductive drums 14 a to 14 d by the use of a toner with one of colors in y , m , c and bk , that is , the latent images is made to be visible . the order of colors is determined by a predetermined order according to an image forming process or a toner property . the toner images formed on the photoconductive drums 14 a to 14 d are transferred primarily to the transfer belt 16 by fitting phases . the transfer belt 16 holds the toner images of colors formed by the developing units 15 a to 15 d in the order of the formed toner images . therefore , a color toner image is formed on the transfer belt 16 . the transfer unit 18 includes a secondary transfer roller 18 a and a secondary transfer backup roller 18 b , and the color toner image is supplied between both of these two rollers by the rotation of the transfer belt 16 . on the other hand , the sheet feeding unit ( sheet cassette ) 12 supplies a recording medium p to the transfer unit 18 . the recording medium p supplied to the transfer unit 18 is also supplied between the secondary transfer roller 18 a and the secondary transfer backup roller 18 b and the color toner image on the transfer belt 16 is secondly transferred to the recording medium p . in fig1 , although the sheet feeding unit 12 is illustrated just one , in general , there are plural sheet cassettes , in which various sizes and types of the recording mediums are contained in the plural sheet cassettes . the recording medium is withdrawn from a sheet cassette selected among the plural sheet cassettes . alternatively , the recording medium inserted from a manual mechanism 22 is supplied to the transfer unit 18 through a medium path 21 . the size of the recording medium corresponds to the size of the color toner image which is formed by the image forming apparatus body 11 . in the transfer unit 18 , the color toner image on the transfer belt 16 is secondly transferred to the recording medium p which is sent through the medium path 21 , and the recording medium p is sent to the fixing unit 19 through a medium path 23 . the fixing unit 19 will be described in detail later . the recording medium p on which the toner image is fixed by the fixing unit 19 is loaded on a stock unit 25 located at a space between the image acquisition unit 13 and the image forming apparatus body 11 through a medium path 24 . the fixing unit 19 has a function of fixing the color toner image on the recording medium p . the perspective view illustrating the structure of the fixing unit 19 is shown in fig2 . the fixing unit 19 includes a heating roller 31 heated , a fixing roller 32 for fixing the toner image , an endless fixing belt 33 wound around the fixing roller 32 and the heating roller 31 , and a pressurizing driving roller 34 press - contacted with the fixing belt 33 by a spring ( not shown ). the recording medium p of which a surface having the toner image is faced to the fixing belt 33 is made to be passed between the pressurizing driving roller 34 and the fixing belt 33 from the bottom to the top . fig3 shows a structure of each roller of the fixing unit 19 and a connection relationship between these rollers and electric circuits thereof . the heating roller 31 has a metal layer on the surface thereof . the fixing roller 32 has a sponge layer with a predetermined thickness on the surface thereof , and a metal core is located therein . the pressurizing driving roller 34 has a rubber layer with a predetermined thickness on the surface thereof , and a metal core is located therein . the heating roller 31 is made of a metal pipe with a 1 mm thickness , the surface thereof is coated in order to prevent rust and improve a lubricating property . the heating roller 31 has a temperature detector 31 s on the outer periphery thereof , and a temperature measured by the temperature detector 31 s is input to an external heating circuit 31 h . the heating circuit 31 h controls heaters 31 a and 31 b located in the heating roller 31 . the pressurizing driving roller 34 also has a temperature detector 34 s on the outer peripheral thereof , and a temperature measured by the temperature detector 34 s is input to an external heating circuit 34 h . the heating circuit 34 h controls a heater 34 a located in the pressurizing driving roller 34 . the heaters 31 a , 31 b , and 34 a are configured with , for example , halogen lamps . the heating roller 31 being heated by the heaters 31 a and 31 b heats the fixing belt 33 and comes to heat the surface holding the color toner image on the recording medium p , where the surface of the recording medium p is press - contacted with the fixing belt 33 when passing between the fixing belt 33 and the pressurizing driving roller 34 . at this time , the pressurizing driving roller 34 heats the surface of the pressurizing driving roller 34 and comes to heat the recording medium p from the rear surface thereof . the surface temperature of the pressurizing driving roller is kept , for example , at 120 ° c . the pressurizing driving roller 34 is connected to a driving motor 35 via a gear , and is rotatably driven by the driving motor 35 . a rotation - speed control circuit 36 controls a rotation speed of the driving motor 35 . the driving motor 35 is a kind of pulse motor whose rotation is controlled by pulses input thereto . the rotation - speed control circuit 36 is connected to a timer 37 . the timer 37 measures a period of time lapsed after starting operation , that is , a warming - up ( wu ) time when the image forming apparatus 10 starts , and inputs the lapsed time to the rotation - speed control circuit 36 . the rotation - speed control circuit 36 has a table as a memory ( not shown ), through which the rotation speed of the pressurizing driving roller 34 is changed according to a period of time after starting operation . specifically , the rotation - speed control circuit 36 includes a speed control memory which has a correspondence relationship of the number of pulses per every minute to be supplied to the driving motor with respect to the time lapsed after the wu time . in a normal fixing in a steady temperature , the pressurizing driving motor is rotated at a rotation speed when 128 pulses per one minute are input to the driving motor 35 . based on the number of pulses ( in this case , 128 pulses ) per one minute in the general case , the number of adding pulses according to a period of time after the wu time may be stored in the speed control memory as a table . results experimented on the image forming apparatus having the above structure will be described . the peripheral surface speed of the fixing belt 33 after starting operation ( wu time ) is measured with respect to an image forming apparatus capable of outputting 35 sheets per one minute and an image forming apparatus capable of outputting 45 sheets per one minute , respectively . the heating roller 31 has a metal layer with a 1 mm thickness on the surface thereof . the fixing roller 32 has a sponge layer with an 8 mm thickness and hardness 350 in asker - c on the surface thereof , and a metal core is located therein . the pressurizing driving roller 34 has an outer diameter of 38 mm , a rubber layer ( hardness 720 in asker - c ) with a predetermined thickness from the surface thereof , and a metal core therein . the rubber thickness on the surface of the pressurizing driving roller 34 is 2 mm in the case of the 35 - sheet machine , and 4 mm in the case of the 45 - sheet machine . in fig4 , the measurement results of the speed in these two kinds of image forming apparatus are shown every one minute after starting operation . in the 45 - sheet machine , a pulse signal of 128 bits / sec corresponding to a normal speed is input to the driving motor 35 from the rotation - speed control circuit 36 . in the 35 - sheet machine , a signal of 131 bits which is larger than 128 bits by 3 bits is supplied to the driving motor 35 for one minute . when the number of pulses is increased by 10 bits , the rotation speed of the pressurizing driving roller rises 0 . 6 %. since 131 bits − 128 bits = 3 bits , the fixing speed in the 35 - sheet machine rises about 0 . 18 % higher than that in the 45 - sheet machine . fig5 shows a graph diagram illustrating the speed data of the 35 - sheet machine and the speed data of the 45 - sheet machine . the horizontal axis indicates a time lapsed after starting operation , and the vertical axis indicates a peripheral surface speed of the transfer belt , this is , a moving speed ( mm / sec ) of the recording sheet in fixing . in fig5 , (♦) indicates data of the fixing speed of the 35 - sheet machine , and the solid line indicates an approximated exponential curve of these data . in fig5 , (▴) indicates data of the fixing speed of the 45 - sheet machine , and the dotted line indicates an approximated exponential curve of these data . in the 35 - sheet machine , the speed when starting operation is 151 mm / sec and changed to about 151 . 5 mm / sec , and then to about 151 . 6 mm / sec . it is understood that , in the 45 - sheet machine , the speed when starting operation is 150 . 4 mm / sec and changed to about 152 mm / sec . from this measurement results , it is possible to understand that , as coming near to the operation start time , the peripheral surface speed of the pressurizing driving roller becomes considerably low . in addition , the thickness of the rubber provided on the surface of the pressurizing driving roller 34 is 2 mm in the case of the 35 - sheet machine , and 4 mm in the case of the 45 - sheet machine . the reason why the thickness of the rubber of the 45 - sheet machine , which is a high - speed machine , is larger than that of the 35 - sheet machine is to increase the contact area ( nip width ) between the rollers . in the 35 - sheet machine , the nip width is 8 mm , and is about 9 . 5 mm in the 45 - sheet machine . in this way , when the nip width of the pressurizing driving roller 34 is made to be increased for speed - up ( for example , from 2 mm to 4 mm ), the speed variation becomes large due to the thermal expansion . therefore , it is necessary to correct the speed . in normal room temperature ( about 20 ° c . ), the pressurizing driving roller 34 has a roller diameter of 38 mm in both the 35 - sheet machine and the 45 - sheet machine , and the peripheral surface speed thereof is 150 mm / sec in both in the normal room temperature . from fig5 , it can be seen that the peripheral surface speed of the fixing belt when starting operation is decreased as the thickness of the rubber on the surface of the pressurizing driving roller 34 increases . that is , it can be understood that the large expansion rate and the high temperature cause the diameter of the pressurizing driving roller 34 to vary largely . further , it can be understood that , if the thickness of the rubber is large , it is difficult to transfer the heat , and it takes much time for the heater 34 a located in the pressurizing driving roller 34 to heat the surface of the roller . from the experimental results , for example , in the 35 - sheet machine , it can be understood that the rotation speed is preferably raised about 0 . 3 % for 10 minutes after starting operation . next , in the fixing unit 19 shown in fig3 , another embodiment in which the rotation speed of the pressurizing driving roller 34 is changed in 2 steps will be described with reference to fig6 . in this case , it is assumed that the rotation speed of the pressurizing driving roller 34 is constant . when the temperature is in a desired constant steady state passing enough time after starting operation , the actual diameter of the pressurizing driving roller 34 comes to be in a predetermined size . at this time , the peripheral surface speed of the pressurizing driving roller 34 is set to vc , and the rotation speed thereof is set to v 0 . when the pressurizing driving roller is made to be rotated at the rotation speed v 0 even though the temperature when starting operation is low , the actual diameter of the roller is small because the temperature is low . therefore , the peripheral surface speed of the pressurizing driving roller 34 is lower than the peripheral surface speed vc . in this case , the rotation speed of the pressurizing driving roller rises up to , for example , v 1 ( v 1 & gt ; v 0 ) by a first predetermined time t 1 after starting operation even though the temperature is low , in order that the peripheral surface speed of the pressurizing driving roller 34 becomes vc . next , the temperature of the pressurizing driving roller 34 increases , but if the temperature does not yet reach to the steady temperature , the peripheral surface speed does not reach to vc . at this time , during a period of time from starting operation to the secondary time t 2 after being lapsed from the first time t 1 , the rotation speed v 2 of the pressurizing driving roller 34 is controlled to be lower than the rotation speed v 1 but higher than the rotation speed v 0 , that is , v 1 & gt ; v 2 & gt ; v 0 , so that the peripheral surface speed almost equals to vc . specifically , the first predetermined time t 1 is 2 minutes , and the second predetermined time t 2 is 15 minutes . the peripheral surface speed v 1 is a speed more increased than normal by 0 . 8 %, that is , corresponds to 128 + 13 = 141 bits . the peripheral surface speed v 2 is a speed more increased than normal by 0 . 4 %, that is , corresponds to 128 + 7 = 135 bits . such being this case , if the rotation speed of the pressurizing driving roller is controlled in 2 steps , the peripheral surface speed of the pressurizing driving roller due to the small roller diameter is corrected and kept almost constant . moreover , it is possible to keep the fixing speed almost constant . as the embodiment , if the rotation speed of the pressurizing driving roller is controlled by being separated in 2 steps , the change in the fixing speed can be suppressed more than in the case of controlling in 1 step of the above embodiment . in the above - mentioned embodiment , the rubber is provided on the surface of the pressurizing driving roller . however , in the invention , it is possible to use a sponge instead of the rubber on the surface of the pressurizing driving roller . further , the rubber or the sponge can be also provided on the surface of the fixing roller . moreover , in the invention , mini - cell rollers can be adopted as these rollers . in the above - mentioned embodiment , it is described about the fixing after the toner image is transferred from the transfer belt to the recording sheet . however , the invention is applicable to the fixing after the toner image is transferred from the transfer roller to the recording sheet . in the above - mentioned embodiment , it is described about the fixing of the color toner image . however , the invention is applicable to the fixing of a monochrome toner image as well as the color toner image . in the above - mentioned embodiment , it is described about that the pressurizing driving roller comes in contact with the rear surface of the recording sheet , that is , that the rotation is driven by the pressurizing roller . however , the invention is also applicable to that the fixing roller which rotatably moves the fixing belt is rotatably driven . in the above - mentioned embodiment , it is described about the fixing using the fixing belt . however , the invention is not limited thereto , and is also applicable to that the fixing roller is directly provided opposite to the pressurizing roller and the recording sheet holding the toner image is moved by being interposed between these rollers without using the fixing roller . in this case , the roller which is driven rotatably may be the pressurizing roller or the fixing roller . in the above - mentioned embodiment , it is described about that the driving motor is a pulse motor and the rotation thereof is changed by varying the number of pulses input to the pulse motor , so that the rotation of the roller driven by the driving motor is changed . however , in the invention , the change in the rotation of the roller may be made without using the pulse motor . obviously , many , modifications and variations of this invention are possible in the light of the above teachings . it is therefore to be understood that within the scope of the appended claims , this invention may be practiced otherwise than as specification .