A light-dimming device, comprising: first and second terminals (1, 2) for receiving an AC voltage; a switch connected between the first and second terminals (1, 2); an adjuster configured to adjust a conduction angle of the switch; a controller configured to control the switch; and a power supply module including a capacitor, said power supply module being connected between the first and second terminals (1, 2) and configured to supply power to the controller, characterized in that the controller includes a detecting circuit configured to detect a voltage across the capacitor, and a determination circuit configured to determine whether or not the voltage across the capacitor detected by the detecting circuit is less than a threshold, the controller is configured to, within a half cycle of the AC voltage, keep the switch off until a first period of time elapses from a point in time of zero-cross of the AC voltage between the first and second terminals (1, 2), turn on the switch when the first period of time elapses and keep the switch on until a second period of time (Ton) having a length corresponding to the conduction angle adjusted by the adjuster elapses from turning on the switch, and turn off the switch when the second period of time elapses from turning on the switch and keep the switch off until an end of the half cycle, and the first period of time has a length equal to or more than zero, and is set to a period of time from the point in time of the zero-cross to a point in time when a voltage value of the voltage across the capacitor becomes greater than or equal to the threshold or more in case the determination circuit determines that the voltage value of the voltage at the point in time of the zero-cross is less than the threshold. first and second terminals (1, 2) for receiving an AC voltage; a switch connected between the first and second terminals (1, 2); an adjuster configured to adjust a conduction angle of the switch; a controller configured to control the switch; and a power supply module including a capacitor, said power supply module being connected between the first and second terminals (1, 2) and configured to supply power to the controller, characterized in that the controller includes a detecting circuit configured to detect a voltage across the capacitor, and a determination circuit configured to determine whether or not the voltage across the capacitor detected by the detecting circuit is less than a threshold, a detecting circuit configured to detect a voltage across the capacitor, and a determination circuit configured to determine whether or not the voltage across the capacitor detected by the detecting circuit is less than a threshold, the controller is configured to, within a half cycle of the AC voltage, keep the switch off until a first period of time elapses from a point in time of zero-cross of the AC voltage between the first and second terminals (1, 2), turn on the switch when the first period of time elapses and keep the switch on until a second period of time (Ton) having a length corresponding to the conduction angle adjusted by the adjuster elapses from turning on the switch, and turn off the switch when the second period of time elapses from turning on the switch and keep the switch off until an end of the half cycle, and keep the switch off until a first period of time elapses from a point in time of zero-cross of the AC voltage between the first and second terminals (1, 2), turn on the switch when the first period of time elapses and keep the switch on until a second period of time (Ton) having a length corresponding to the conduction angle adjusted by the adjuster elapses from turning on the switch, and turn off the switch when the second period of time elapses from turning on the switch and keep the switch off until an end of the half cycle, and the first period of time has a length equal to or more than zero, and is set to a period of time from the point in time of the zero-cross to a point in time when a voltage value of the voltage across the capacitor becomes greater than or equal to the threshold or more in case the determination circuit determines that the voltage value of the voltage at the point in time of the zero-cross is less than the threshold.