An optical sensor detects at least one of a toner image and a toner adhesion amount per unit area based on a change of an optical characteristic on a surface of an endless belt member. A control unit performs a predetermined process based on a detection result obtained by the optical sensor. A sensor moving unit moves the optical sensor between a first position in opposite to a belt supporting area in which the endless belt member is supported by a supporting roller and a second position that is different from the first position. The detection result is obtained by the optical sensor when the optical sensor is placed at the first position by the sensor moving unit.