An AC power supply source switching apparatus includes a switch and a switch connecting a load to an AC power supply system and a DC secondary battery, respectively. When one switch, which is driven to turn off, did not actually turn off, the switch is driven to turn off again after being turned on. When an impedance of the load decreases and a load current flows in a load line, no AC voltage is developed on the load line. As a result, it is determined that the switch has actually turned off. Then the other switch is turned on thereby to switch over connection of the load.