To run an intermediate transfer belt at a constant speed regardless of eccentricity in a driving roller, etc., or disturbance.

An original image is written onto photoreceptor drums 10 of four colors by an exposing device 7, developed by a developing apparatus 11 and transferred to the intermediate transfer belt 12 by a transfer roller 13. The transferred toner image is transferred to transfer paper by a transfer roller 17. The intermediate transfer belt 12 is hung between a driving roller 14, a roller 15 and a following roller 16 and two or more position detectors (position marks) are provided in a core metal part of the following roller 16 at approximately equal intervals and light passing through the intermediate transfer belt 12 is detected by an optical sensor 18. Time for one rotation of the following roller is measured for each position mark by this detection. A storing part of a control apparatus 3 stores a reference rotational time for one rotation according to the position marks, a driving motor 4 is controlled by the difference from measured rotational time and the rotational speed of the driving roller 14 is controlled through a speed reduction gear 5. PID control is carried out with the outer periphery of the driving roller as an integral multiple of the outer periphery of the following roller.