PURPOSE: To improve the reliability of a system comprehensively, by stopping the working of a controlling circuit with a fault in case that the controlling circuit gets out of order, and after that, by controlling an AC motor continuously with a standby controlling-circuit.

CONSTITUTION: At the internal section of a controlling circuit section 20, either one of redundant-system-composed controlling circuits 22 and 23 is used at the time of operating the controller of an AC motor, and the controlling circuits 22 and 23 are controlled by operating circuits 221, 231 so that the other controlling circuit 22 or 23 may be in a standby state. When a fault is generated in the controlling circuit 22 or 23 under operation in such a state, then the fault is detected by the operating circuit 221 or 231, and the controlling circuit 22 or 23 under operation is stopped, and via the other operating circuit 221 or 231, the operation of the other controlling circuit 22 or 23 is started.