1. An address determining method for determining, in a lighting system including luminaires, image sensors which perform lighting control for the luminaires, and a controller which gives the image sensors an instruction regarding lighting of the luminaires, a positional relationship among the luminaires and the image sensors, wherein when the controller receives an external instruction for initial setting, the controller gives the image sensors the instruction regarding lighting, when an image sensor among the image sensors receives the instruction regarding lighting from the controller, the image sensor performs lighting control in accordance with the instruction regarding lighting on a luminaire among the luminaires to determine (i) a relative position of the luminaire relative to the image sensor and (ii) a relative position of the image sensor relative to a different image sensor among the image sensors, and the controller determines positions of the image sensors and positions of the luminaires by collecting the relative positions of the image sensors and the relative positions of the luminaires determined by the image sensors.