PURPOSE: To provide a system for estimating disturbance torque in a control system having variable torque constants.

CONSTITUTION: In a disturbance torque observer for a motor, an input flux command to the motor passes through a primary filter in order to calculate an estimated flux value of the motor. Torque constants in the disturbance torque observer is then varied (Step S2) based on the estimated flux value thus estimating a disturbance load torque.