An input shaft speed signal for rotating a motor at a predetermined rotational speed is output from a rotation control unit to the motor. A rotational speed reduced by a speed reduction device with elastic bodies is detected by an output rotation sensor. A CPU obtains slippage by calculating a reduction ratio based on the detected output shaft speed signal and input shaft speed signal. Load torque is determined from the obtained slippage. A current value to be supplied to the motor is determined so that the motor produces torque corresponding to the load torque. The current value to be supplied from a motor driver to the motor is thus controlled.