PURPOSE: To cause no overcurrent to flow through a motor at the time of restarting by outputting the output with an output frequency similar to that of a motor terminal voltage and with a phase coinciding with that of said voltage from a power inverter at the time of service recovery.

CONSTITUTION: When a power-failure detection circuit 100 detects a recovery of service, a pulse signal outputted from a residual magnetic flux detection circuit 200 is directly inputted to a microprocessor 12. Then, the number of revolutions of a motor 5 is operated from said pulse signal by a first arithmetic means, and the phase control signal of an inverter output voltage corresponding to the phase of said pulse signal is obtained by a second arithmetic means. The microprocessor 12 controls a power inverter 4 via a base drive circuit 6 on the basis of said operated number of revolutions of the motor, phase control signal and direction of phase rotation stored in ROM 11.