PURPOSE: To prevent such a fact that control over an invertor is continued and voltage in a control circuit goes up due to generative power from an electric motor in time of cutting off the power, in this invertor performing its power regeneration.

CONSTITUTION: A closing coil 23 of a safety circuit 22 is magnetized at an auxiliary contact 2a2 of a molded case circuit breaker 2 connected to a three- phase power source, and at this auxiliary contact 23a, both operating and braking coils of a main circuit electromagnetic contactor contacted between the molded case circuit breaker 2 and a converter are demagnetized.