PURPOSE: To simplify and to transmit desired information by transmitting a DC plus signal and a DC minus signal and an AC signal of specific frequency through a signal transmission line in a superposition state, and combining those three signals selectively.

CONSTITUTION: When the changeover contact A1 of a station A is operated to connect a branch line L1' to a DC plus power source terminal B and a branch line L2' to a DC minus power source terminal C, a DC plus signal which holds a signal L1 at a plus potential and a signal line L2 at a minum potential is set out. When a changeover contact A2 is operated, the polarity is inverted and a minum DC signal is sent out. When a changeover contact A3 is operated, L1" and L2" are connected to AC power source terminals BX and CX to send out the AC signal of specific frequency to the lines L1 and L2. Further, when the contacts A1 and A3 are operated at the same time, the superposition signal of the DC plus signal and signal (f) is sent out, and when the contacts A2 and A3 are operated, the superposition signal of the DC minus signal and signal (f) is sent out.