To enhance reliability with the minimum configuration by reducing the interference of the calling signal of a transmission signal and the adverse effect of the external noise on a sensor line on the transmission signal.

A transmission line S, a transmission common line SC, a power line V, a telephone line T, and a common line C with respect to the power line V and the telephone line T are pulled out as transmission lines 2 from a receiver 1. A repeater 3 for a sensor comprises a transmission circuit section 13, a CPU (Central Processing Unit) circuit section 14, and a fire reception circuit section 15. The transmission circuit section 13 is connected to the CPU circuit section 14 through electrically isolating photocouplers 16, 17. The transmission circuit section 13 is connected to the transmission line S and the transmission common line SC from the receiver 1. The CPU circuit section 14 and the fire reception circuit section 15 side are connected to the power line V and the common line C from the receiver 1. In addition, the telephone line T from the receiver 1 is connected to one end of a telephone jack 5; and a telephone common line TC from the other end of the telephone jack 5 is connected to the common line C from the receiver 1.