PURPOSE: To miniaturize a power-feeding circuit for directly feeding commercial power supply by feeding power via an inverter normally and then bypassing it on overloading and then reduce its cost.

CONSTITUTION: The voltage of a commercial power supply is detected via a transformer 9, the detection voltage value and a current value which is detected via a current transformer 3B are input to a divider 10 to obtain load impedance, the load impedance is compared with the set value from a setting equipment 7B to check to see if overloading exists, and then change-over is made for overloading. The judgment can also be made according to the amount of load power instead of load impedance.