Belt driving device and image forming apparatus

An image forming apparatus comprising:

• A belt driving device including
• an endless belt member,
• a plurality of supporting rollers over which the endless belt member is supported, the supporting rollers include at least a driving roller that drives the endless belt member and a driven roller that is driven by a rotation of the endless belt member,
• a driving unit that drives the driving roller,
• a plurality of marks provided on the driven roller,
• a detecting unit that detects the marks on the driven roller at a predetermined position, and
• a control unit that controls a speed at which the driving unit drives the endless belt member based on a result of detecting the marks by the detecting unit, wherein
• the driven roller functions as a tension roller that applies a tension to the endless belt member, and
• the detecting unit is held by the tension roller;
• an image carrier that carries an electrostatic latent image;
• a developing unit that develops the electrostatic latent image on the image carrier to obtain a visible image;
• and
• a transferring unit that transfers the visible image on the image carrier directly to a recording medium that is held on a surface of the endless belt member, or transfers the visible image on the image carrier to the endless belt member and transfers the visible image on the endless belt member to the recording medium.