PURPOSE: To decrease the probability of line interruption by discriminating whether or not a transmitter not selected is normal when the selected transmitter is faulty and switching the transmitter when normal so as to eliminate the interference of the transmitters and reduce power consumption.

CONSTITUTION: Terminals 4a, 4d of a switch 4 are connected normally and a signal at a terminal A is transmitted via a transmitter 1. When the transmission level of the transmitter 1 is lower than a prescribed value, the switch 4 transmits a "1" signal from the terminal 4b, and as a result a controller 5 transmits a signal turning on the power supply to the transmitter 3 and a logical circuit 7 and the transmitter 3 transmits a signal of the terminal A. When the output level of the transmitter 3 is larger than a prescribed value, a detector 6 applies the "1" signal to the logical circuit 7 and the circuit 7 applies the "1" signal to the switch 4. As a result, the connection between the terminals 4a, 4d of the switch 4 is disconnected and terminals 4e and 4d are connected. In this case, when the transmission level of the transmitter 3 is a prescribed value or below, since the detector 6 transmits no "1" signal, the switch 4 does not execute the said switching, then the line is kept with an output signal of the transmitter 1 regardless of an insufficient value.