1. An illumination system, comprising: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 a brightness of the one or more luminaires and including a communicator that wirelessly communicates with the plurality of luminaires,
wherein the controller:
divides the plurality of luminaires into a plurality of groups to classify, into a same group, luminaires among the plurality of luminaires that are associated with a same address; 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.