A power supply is electrically connected between a pair of input terminals and is configured to be supplied with electric power from an AC power supply to generate control electric power. A voltage detector is configured to detect a voltage of the control electric power. A controller is configured to be supplied with the control electric power from the power supply to operate. The controller is configured to control a bidirectional switch based on a detection signal from a phase detector so as to switch the bidirectional switch from an ON state to an OFF state at a switching time point when a variable time corresponding to a dimming level elapses from a starting point of a half period of an AC voltage. A stopper is configured to halt generation of the control electric power by the power supply when the voltage detected by the voltage detector is higher than or equal to a prescribed threshold during an OFF time period from the switching time point to an end time point of the half period.