A plurality of support rollers rotatably support a belt member. A first motor and a second motor each rotate one of the plurality of support rollers. In a normal mode, a normal control portion derives a load index value based on information on a load torque of the plurality of support rollers, and controls a second supply power supplied to the second motor based on power control data representing a correspondence between the load index value and a motor supply power. In a calibration mode, a calibration control portion corrects the power control data by comparing a speed detection value when a test drive control is being executed or a test actual value which is a power supplied to the second motor with a reference value.