A transmitting section 7a outputs a transmission signal to the side of a transmission line 1. A first switching section Qa1 outputs the transmission signal to the transmission line 1. A second switching section Qa2 outputs the transmission signal from the transmission line 1 . A receiving section 9a receives the transmission signal from the transmission line 1. A first detecting section 13a detects the transmission signal flowing through the first switching section Qa1. A second detecting section 19a detects the transmission signal flowing through the second switching section Qa2. When the transmission signal from the transmitting section 7a is not detected at both the first and second detecting sections 13a and 19a, a selecting section 15a selects the receiving section 9a and outputs a reception signal.
