A sensor device according to an example of the embodiment is connected, on an input side rather than an illumination apparatus, to a main power source line of an illumination system to which a DC voltage is applied and to which the illumination apparatus is connected through a branch wire. The sensor device includes: a sensing unit that detects an object and outputs a detection signal; and a switch that is provided in the main power source line and connects or disconnects the main power source line by an on-off operation on the input side rather than the illumination apparatus. The switch is controlled to be turned on or off based on the detection signal of the sensing unit.