PURPOSE: To eliminate adjusting operation by operating the nominal value of viscous resistance from the output of a PI control part and the speed of an electric motor when the output of a feedforward control part and the output of the speed PI control part are less than set threshold values.

CONSTITUTION: An equivalent disturbance estimating unit 3 operates an equivalent disturbance estimated value Tais. The output Upi of a proportional and integral controller 9, the output Tdis of the equivalent disturbance estimating unit 3, the output of the feedforward control part 10, and the electric motor speed are inputted to a machine constant estimation part 2. Judging from an operation state, a nominal value Jn of inertia, a nominal value Dn of viscous resistance, and a nominal value Ktn of the torque generation coefficient of a power actuator are operated. Those estimated values as the output 4 of the machine constant estimation part 2 are used to update respective values in a nominal multiplier 5 for the torque coefficient, a differentiator 7 which calculates a nominal value of the inverse function of a control system, the proportional and integral controller 9, and the feedforward control part 10. Consequently, a high-performance control system can be constituted.