A controller is configured, by decreasing light quantity in only a first slot that is any one of a plurality of slots into which a unit time is divided equally, and increasing light quantity in all the remaining slots except the first slot of the plurality of slots, to assign information to the first slot of the plurality of slots. The controller is furthermore configured to decrease light quantity in second slots that are three or more continuous slots starting from a head of a preamble, which indicates a frame start point, of a frame of an optical-communication signal and to increase light quantity in a slot immediately before the preamble.