A modulation device includes a modulator circuit that causes a light source to perform light communication, storage storing an internal ID, and a control circuit that determines whether an external ID is input into the modulation device. When the control circuit determines that the external ID is input into the modulation device, the control circuit causes the input external ID to be input, as a modulation signal, into the modulator circuit. When the control circuit determines that the external ID is not input into the modulation device, the control circuit causes the internal ID stored in the storage to be input, as a modulation signal, into the modulator circuit.