Abstract: A switch is connected between an AC power supply and a load. A controller detects an abnormality of the AC power supply by detecting an instantaneous value of a three-phase AC voltage supplied from the AC power supply, when the switch is on. Based on an instantaneous value of the three-phase AC voltage detected at a first time and a first threshold value preset for a peak value of the three-phase AC voltage, the controller estimates a second threshold value for an instantaneous value of the three-phase AC voltage at a second time having a prescribed time difference from the first time. The controller detects the abnormality of the AC power supply by comparing the estimated second threshold value with the instantaneous value of the three-phase AC voltage detected at the second time. The controller turns off the switch when the abnormality of the AC power supply is detected.