To detect service interruption quickly and stably by determining service interruption of commercial power source when a voltage waveform lowers below a set voltage value and the time of lower voltage exceeds a set time.

A service interruption detection circuit 7 performs full-wave rectification of an alternating current voltage waveform from a commercial power source 1. On the other hand, a set voltage value is inputted from a setting device 9 so as to be stored in the detection circuit 7. The detection circuit 7 determines whether the fully rectified voltage waveform is lower than a set voltage (30 V for example) or not and whether the time of the lower voltage is longer than a set time (3.3 ms for example) or not. When the commercial power source voltage waveform is lower than the set voltage value, the time of the lower voltage is counted, and service interruption is determined if the time exceeds the set time. In this case, the detection circuit 7 actuates an inverter 6 for converting direct current from a storage battery 5 into alternating current, and at the same time, switches a switch 2 from commercial power source supply to inverter power supply for feeding alternating power to a load 3. Then, when the voltage waveform is increased above 30 V, time counting is canceled.