A relay device comprises: a first switch which is inserted between a first extraction unit and a second superposition unit; and a second switch which is inserted between a second extraction unit and a first superposition unit. The first switch switches on and off the routing of a superposition signal which is relayed from a first transmission path to a second transmission path, and the second switch switches on and off the routing of a superposition signal which is relayed from the second transmission path to the first transmission path. The first switch (1) receives a first control signal and is controlled to switch on and off so as to be switched off when the superposition signal is relayed from the second transmission path to the first transmission path. The second switch receives a second control signal and is controlled to switch on and off so as to be switched off when the superposition signal is relayed from the first transmission path to the second transmission path.
