When an operator operates an operation tool of a dimming operation terminal device in order to set the brightness of a lighting load to desired brightness, an energization electric signal, for example, a voltage level detected by a dimming control section changes. The dimming control section generates a dimming signal modulated according to the voltage level. Power supply to the dimming control section is performed by an insulated auxiliary power supply. Transmission of the dimming signal is performed via an insulating section.