A relay device includes: a housing including: a first surface; and a second surface which is different from the first surface; a light signal receiving element operable to receive a light signal from outside, the light signal receiving element provided at the first surface of the housing; a light emission element operable to emit light responsive to the light signal received by the light signal receiving element; and a longitudinally elongated light guide member provided at the second surface of the housing, the light guide member to which the light emitted from the light emission element is incident, the light guide member including an emission part which causes the incident light to be emitted to outside from a lateral surface of the light guide member, the lateral surface which extends along a longitudinal direction of the light guide member.