PURPOSE: To eliminate the discontinuation of a voltage generated when the power supplying route of the supply device is switched from a route via DC/AC- conversion section to a directly supplying by-pass route by means of a change- over switch by setting an AC/DC-converting section to a stopping state.

CONSTITUTION: An AC/DC-conversion section 1 converts commercial power received through an input section 7 into a direct current and a filter section 2 smoothes the direct current. The smoothed direct current is supplied to a load from an output section 8 through a change-over switch 6 after the current is again converted into the AC power by means of a DC/AC-conversion section 4. When the commercial power supply is interrupted, the DC power of a battery 3 is supplied to the section 4 so as to conduct the load. At the time of performing maintenance or inspections or when a fault occurs, the power is supplied to the load through a directly supplying by-pass section 5 by switching the switch 6, but, at the time of switching the switch 6, the section 1 is controlled to a stopping state. Therefore, the output voltage of the section 8 can be maintained in the state of a sine wave having no discontinuation and the malfunction of the load can be prevented.