To provide a dimmer that can prevent increase of the number of types of dimmers due to different load capacities and different specifications of operating parts, and also reduce heat generation in the operating part.

It comprises an operating part for getting an illumination level of illumination load L between input terminals T1, T2 of the dimmer A and is connected to a series circuit connecting the dimmer B generation a phase control signal according to the operation of operating part with a utility alternating current power supply AC. A series circuit connects the illumination load L with the utility alternating power supply AC between load terminals T3, T4 of the light control unit A. In the unit A, the phase control circuit part 40 controls the phase of the illumination load L according to the phase control signal inputted from the input terminals T1, T2 and lights up the illumination load L at a predetermined lighting level.