To adjust illuminance of a mercury lamp without lighting or putting out the lamp, and to save electric power by providing a light dimming device capable of adjusting illuminance by changing mercury lamp impression voltage, between the mercury lamp and the ballast.

A light dimming device 1 composed of a transformer 4, a switch 5 and a controller 6 is provided between a ballast 8 and mercury 2, Here, the transformer 4 is provided with plural taps 3 of a single winding on the primary side. An output terminal of the ballast 8 and a single tap are connected together in the switch 5 controlled to be opened/closed by the controller 6. The controller 6 has a timer circuit and a CPU programing a procedure to change illuminance by changing impression voltage of a mercury lamp 2 by being switched to a prescribed tap according to the time. Illuminance can be easily obtained when previously finding the relationship between secondary voltage of the transformer 4 and the illumination. Thus, since voltage is changed in the rear stage of the ballast 8, illuminance/light dimming control and electric power consumption-saving can be performed without influencing the mercury lamp lighting action.