PURPOSE: To prevent a compressor or other components from damaging or breaking by shifting the operation control of the compressor to a safety side when an operating current is not the lowest current value or higher to avoid an overcurrent.

CONSTITUTION: When an operating current detected by a current detector 4 is not the lowest current or higher under all operating condition, a current detector 4 malfunctions due to a disconnection or a short-circuit, etc. Even if the operation of a compressor 2 is continued, it can be so controlled that the operating current falls within the highest allowable range until a CPU 9 is updated on the basis of a detected temperature so that an operating frequency instructed arrives at a critical frequency.