Light-dimming device includes: first and second terminals; switch including switching device connected between terminals; adjuster for varying conduction angle of switch; controller for controlling switch and power supply module for supplying power to controller. Controller includes zero-cross detection circuit for detecting zero-cross of AC voltage, control circuit for generating PWM signal of on-duty ratio corresponding to conduction angle from adjuster, and driver circuit for turning device on and off by PWM signal. Controller allows device to conduct and then turn off within variable period of time, from start of half cycle of AC voltage, shorter than half cycle. Circuit starts generation of pulse in PWM signal when prescribed period of time, shorter than variable period of time, from zero-cross of voltage elapses.