A light-dimming device, comprising: first and second terminals (1, 2) that allow a series circuit of an AC power supply and a lighting load to be connected between; a switch that comprises a normally open switching device connected between the first and second terminals (1, 2); an adjuster that is configured to adjust a conduction angle of the switch; a controller that is configured to control the switch and that comprises a zero-cross detection circuit that is configured to detect zero-cross of an AC voltage (Va) from the AC power supply with the series circuit connected between the first and second terminals (1, 2), a control circuit that is configured to generate a PWM signal that defines an on-duty ratio corresponding to the conduction angle acquired from the adjuster, and a driver circuit that is configured to turn the switching device on and off in accordance with the PWM signal; and a power supply module that is connected between first and second terminals (1, 2) and configured to supply power to the controller, characterized in that the power supply module comprises: a rectifier circuit that is configured to convert the AC voltage (Va) into a full-wave rectified voltage; a capacitor; and a constant voltage circuit that is configured to generate a first DC voltage from the full-wave rectified voltage by the rectifier circuit to apply the first DC voltage across the capacitor, the controller is configured to control the switch by allowing the switching device to conduct and then turn off within a variable period of time, from a start of each half cycle of the AC voltage from the AC power supply, which is shorter than a corresponding half cycle, and the control circuit is configured to start generation of a pulse, to be contained in the PWM signal, for turning the switching device on when a prescribed period of time, which is a period of time while the capacitor is charged to a potential difference of the first DC voltage, elapses, and to stop the generation of the pulse at an end of the variable period of time, the prescribed period of time being a period of time, shorter than the variable period of time, from detection of the zero-cross of the AC voltage (Va) by the zero-cross detection circuit. first and second terminals (1, 2) that allow a series circuit of an AC power supply and a lighting load to be connected between; a switch that comprises a normally open switching device connected between the first and second terminals (1, 2); an adjuster that is configured to adjust a conduction angle of the switch; a controller that is configured to control the switch and that comprises a zero-cross detection circuit that is configured to detect zero-cross of an AC voltage (Va) from the AC power supply with the series circuit connected between the first and second terminals (1, 2), a control circuit that is configured to generate a PWM signal that defines an on-duty ratio corresponding to the conduction angle acquired from the adjuster, and a driver circuit that is configured to turn the switching device on and off in accordance with the PWM signal; and a power supply module that is connected between first and second terminals (1, 2) and configured to supply power to the controller, characterized in that the power supply module comprises: a rectifier circuit that is configured to convert the AC voltage (Va) into a full-wave rectified voltage; a capacitor; and a constant voltage circuit that is configured to generate a first DC voltage from the full-wave rectified voltage by the rectifier circuit to apply the first DC voltage across the capacitor, a rectifier circuit that is configured to convert the AC voltage (Va) into a full-wave rectified voltage; a capacitor; and a constant voltage circuit that is configured to generate a first DC voltage from the full-wave rectified voltage by the rectifier circuit to apply the first DC voltage across the capacitor, the controller is configured to control the switch by allowing the switching device to conduct and then turn off within a variable period of time, from a start of each half cycle of the AC voltage from the AC power supply, which is shorter than a corresponding half cycle, and the control circuit is configured to start generation of a pulse, to be contained in the PWM signal, for turning the switching device on when a prescribed period of time, which is a period of time while the capacitor is charged to a potential difference of the first DC voltage, elapses, and to stop the generation of the pulse at an end of the variable period of time, the prescribed period of time being a period of time, shorter than the variable period of time, from detection of the zero-cross of the AC voltage (Va) by the zero-cross detection circuit.