In order to reduce distortion of real-current wave form and to suppress substantial transient fluctuation of the current and torque during changeover from the regeneration running to the power running of a three-phase induction motor (66), changeover from the three-phase-modulation control to the two-phase-modulation control is carried at the same time the power running of the motor (66) is changed to the regeneration running according to a motor-state-quantum such as a detected slip ratio, detected torque, detected current or the like. Thus, distortion of the motor-current wave form is reduced and significant fluctuation of the motor-current and torque during the changeover is reduced. <MATH>
