A relay device includes a first transmitting and receiving unit connected to a PON line on the upper-level side and performing mutual conversion between optical and electrical signals; a second transmitting and receiving unit connected to a PON line on the lower-level side and performing mutual conversion between optical and electrical signals; a relay processing unit relaying a downstream frame received by the first unit to the second unit, and relays an upstream frame received by the second unit to the first unit; and a control unit. The control unit follows upstream multiple access control performed by a station side device, for transmission of the upstream frame to be transmitted by the first unit to the station side device, and independently performs upstream multiple access control for reception of the upstream frame received by the second unit from a home side device.