To stabilize an estimated value for disturbance torque output even if the positional information is obtained from an encoder having the definite bit length, by correcting the output of an observer with the final value of an output transfer function which is inputted with the angular position as a step input when the encoder generates a carry signal or a borrow signal.

A disturbance observer of a control system uses the output θm of an encoder which converts the angle of rotation of a rotor to a digital signal, as the positional information for a motor. In this observer, Gopinath minimum dimension observer is established with estimated disturbance torque Fdis and an estimated speed of a motor ωm. When a carry signal (carry flag) or a borrow signal (borrow flag) is generated, transfer functions Z1, Z2 are reduced by correction values k1, k2, respectively. By this method, the estimated disturbance torque Fdis caused by the discontinuity of the output signal θm of the encoder which is outputted by the disturbance observer can be stabilized.