A dimming device comprising: a switch section configured to be connected to an alternating-current power supply in series to a load and configured to phase-control an alternating-current voltage supplied to the load; a synchronization-signal generating section configured to generate a synchronization signal synchronized with an alternating-current voltage waveform of the alternating-current power supply; a control power supply section connected to the switch section in parallel and configured to convert the alternating-current power supply into a predetermined control power and enable control of operation and stop of a converting action wherein the control power supply section include a capacitive element configured to accumulate the control power; and a control section configured to receive supply of the control power from the control power supply section through the capacitive element and divide, on the basis of the synchronization signal generated by the synchronization-signal generating section, a period of each half cycle of the alternating-current voltage into a first section, a second section and a third section, wherein the control is further configured to: in the first section, subject the switch section to conduction control to supply electric power to the load and subject the converting action of the control power supply section to stop control, in the second section, subject the switch section to non-conduction control to interrupt the power supply to the load and subject the converting action of the control power supply section to stop control, and in the third section, subject the switch section to the non-conduction control to interrupt the power supply to the load and subject the converting action of the control power supply section to operation control. a switch section configured to be connected to an alternating-current power supply in series to a load and configured to phase-control an alternating-current voltage supplied to the load; a synchronization-signal generating section configured to generate a synchronization signal synchronized with an alternating-current voltage waveform of the alternating-current power supply; a control power supply section connected to the switch section in parallel and configured to convert the alternating-current power supply into a predetermined control power and enable control of operation and stop of a converting action wherein the control power supply section include a capacitive element configured to accumulate the control power; and a control section configured to receive supply of the control power from the control power supply section through the capacitive element and divide, on the basis of the synchronization signal generated by the synchronization-signal generating section, a period of each half cycle of the alternating-current voltage into a first section, a second section and a third section, wherein the control is further configured to: in the first section, subject the switch section to conduction control to supply electric power to the load and subject the converting action of the control power supply section to stop control, in the second section, subject the switch section to non-conduction control to interrupt the power supply to the load and subject the converting action of the control power supply section to stop control, and in the third section, subject the switch section to the non-conduction control to interrupt the power supply to the load and subject the converting action of the control power supply section to operation control.