To prevent the occurrence of color shift or the like in images of two or more colors put one over another regardless of the existence of a defective part graduated at non-uniform intervals of a scale on an transfer belt.

A scale 5 having many graduations is provided on an intermediate transfer belt 10 throughout in the peripheral direction, and the scale 5 is read by a sensor 6, and a controller 70 controls the speed of the intermediate transfer belt 10 to a target speed on the basis of an output from the sensor 6. The scale 5 has a defective part 11 being, for example, a joint, and image formation on the intermediate transfer belt 10 is avoided while the sensor 6 detects the defective part 11. In this case, the sensor 6 is arranged in a primary transfer position H of a photoreceptor 40Y in a first station or in the vicinity of the downstream side of the primary transfer position H in order to use an effective image formation area on the intermediate transfer belt 10 most widely.