To suppress backlash oscillation by executing the estimation arithmetic operation of a motor speed according to an equivalent inertia value obtained by adding the inertia of a motor and a gear backlash part and a torque command, multiplying the deflection between this arithmetic value and a motor speed by the nominal value of the viscosity resistance of the backlash part, and operating feedback.

In a ternary inertia control system including backlash elements such as a gear and a coupling, a motor inertia Jm and a gear backlash part inertia Jg are added so that an equivalent Jmg can be obtained. Next, a backlash compensating part 12 calculates an estimated arithmetic value ωg' of a motor speed from a torque command T* and the equivalent inertia value Jmg' which are the input of a motor 2, multiplying a speed deflection ωm-ωg' between the estimated arithmetic value ωg' and the motor speed ωm by a nominal value Dvh of the viscosity resistance of the gear backlash part, and feedbacks it to the torque command T*. Moreover, it is desirable that the variable control of the nominal value Dvh of the viscosity resistance of the gear backlash part is operated according to the size of this speed deflection ωm-ωg'.