BELT DRIVING DEVICE AND IMAGE FORMING APPARATUS

PROBLEM TO BE SOLVED: To provide a belt driving device which can perform highly accurate speed control of an endless belt when performing image forming by using a plurality of image carriers and can prolong the life of the endless belt without increasing the size of an image forming apparatus.SOLUTION: A first contact/separation bracket 151 of first belt contact/separation means 150 which makes a photoreceptor 40K arranged at the most downstream side in a belt moving direction and an intermediate transfer belt 10 contact with and separate from each other holds an upstream side support roller 153, a first transfer roller 62K and a downstream side support roller 154 from the upstream side in the belt moving direction in this order. It is configured in such a manner that when the intermediate transfer belt 10 is stretched across the rollers which are held by it, the course of the belt becomes approximately planar. When performing image forming by using a plurality of photoreceptors 40, a belt-facing surface 131 of a sensor holding member 130 held by sensor moving means 160 in a movable manner is brought into contact with the lower surface of the intermediate transfer belt 10 while being in parallel with it in a section between the upstream side support roller 153 and the first transfer roller 62K.