An illumination system includes: a plurality of luminaires; and a controller including a brightness sensor that is used for light intensity control on one or more luminaires among the plurality of luminaires and detects the brightness of the one or more luminaires. The controller divides the plurality of luminaires into a plurality of groups and performs the light intensity control in a first mode or a second mode for each of the plurality of groups. The first mode is a mode in which the light intensity control is performed according to a result of detection by the brightness sensor, and the second mode is a mode in which the light intensity control is performed according to an instruction on a light intensity rate provided from outside.