PURPOSE: To eliminate a DC braking without moving a motor shaft by providing a phase detector for detecting time when one of three phase PWM signals of a PWM waveform forming circuit becomes 0V, a logic circuit, etc.

CONSTITUTION: In the case of a decelerating drive, an operating frequency reaches a DC braking operation frequency fB, and when a phase detector 11 detects time when one of three phase PWM signals of a PWM forming circuit 7 becomes 0V, the conditions of an AND circuit 12 are satisfied, and a DC braking operation frequency detector 9 outputs a detection signal. An inverter main circuit 2 applies, when receiving the signal, a predetermined voltage to U-, V-phases of an induction motor 3 for predetermined time to supply dC currents IU, IV thereby to generate braking torques, thereby performing a DC braking. Magnetic flux vectors ΦU, ΦV of the currents IU, IV become reverse vectors, and a rotary magnetic flux ΦU,V,W of the synthetic vector does not alter its direction. Therefore, the motor shaft of the motor 3 does not move at this time.