A relay device is one of relay devices each including (i) a common port connected to a terminal and (ii) two ring ports connected to other relay devices via communication lines. Upon receiving via the common port a frame whose destination is a terminal connected to a different relay device under one of the communication lines being abnormal, the relay device transmits the frame via both (i) one of the ring ports bypassing the abnormal communication line and (ii) a communication bus. In contrast, upon receiving a frame, whose destination is a terminal connected with the relay device, via one of the ring ports and a frame, whose destination is the terminal connected with the relay device, via the communication bus, the relay device determines whether the frames match. If the frames match, the relay device transfers the matched frame to the terminal via the common port.