A plurality of supporting rollers over which the endless belt member is supported includes 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 detecting unit detects a plurality of marks provided on the driven roller at a predetermined position. A control unit controls a speed at which a driving unit drives the endless belt member based on a result of detecting the marks. The driven roller functions as a tension roller that applies a tension to the endless belt member. The detecting unit is held by the tension roller.