PURPOSE: To supply a stable current to a load by calculating a current to be output from an inverter, setting a predetermined band width to the current, and controlling the current of the inverter within the band width.

CONSTITUTION: When an inverter 6 is operated synchronously with a commercial power source 1 and the power source 1 is switched to an inverter 6 power supply, a current i1 to be output from the inverter 6 is calculated, and the current of the inverter 6 is controlled within a predetermined band width ±i with respect to the current i1. Thus, since the current of the inverter 6 does not exceed the predetermined band width ±i with respect to the current i1 to be output from the inverter 6, an excess current does not flow between the power source 1 and the inverter 6, and hence a variation in an output thereby is small.