To prevent runaway of an inverter and application of an overvoltage to a load machine by making a decision that a voltage detector is abnormal when the difference between a voltage detection value and a set level exceeds a decision level thereby stopping operation of an inverter.

If an abnormality due to damage occurs in a voltage detector when a difference 16 is inputted from a subtractor 14 to a decision circuit 18, the difference 16 increases gradually. When the difference 16 exceeds a decision level 17 (inverter output voltage corresponds to 220 V at that moment), the decision circuit 18 makes a decision that a voltage detector 5 is abnormal and stops operation of an inverter 2 by delivering a stop signal 19. Consequently, AC output voltage VAC is prevented from increasing due to runaway of the inverter 2 thus preventing application of an overvoltage to a load machine 4.