A scale is provided along at least one side of a portion of a belt. A sensor reads the scale on the belt to obtain scale information. An actual speed calculating unit calculates a speed of the belt from the scale information. A speed calculating unit calculates a speed of the belt from information other than the scale information. A control unit that provides a control to correct speed of the belt according to the speed calculated by the actual speed calculating unit when the speed calculated by the actual speed calculating unit is normal, and provides a control to correct speed of the belt according to the speed calculated by the speed calculating unit when the speed calculated by the actual speed calculating unit is abnormal.