PURPOSE: To detect an overload state accurately and protect an inverter circuit which is used in a miniature electric machine as a power supply means for a load from breakdown.

CONSTITUTION: A voltage dividing resistor is connected in parallel to a secondary battery which is charged by an inverter circuit 22 and a switching device 74 is connected in series to the voltage dividing resistor. By the on-off control of the switching device 74 with the output voltage of the inverter circuit 22, if the switching device 74 is turned on, a low voltage which is the divided voltage of a terminal voltage can be taken out and, if the switching device 74 is turned off, the terminal voltage can be taken out as it is as a reference voltage. A detected voltage whose magnitude corresponds to a load value is compared with the reference voltage and, if the detected voltage is below the reference voltage, the operation of the inverter circuit 22 is stopped.