PURPOSE: To keep stable communication quality without talking loss by inserting a transmission coupling member in series with a telephone line at transmission, connecting a reception coupling member in parallel in terms of AC, short- circuiting the transmission coupling member in the standby state and keeping the impedance between telephone set side terminals to the same state as a line impedance.

CONSTITUTION: When a communication start signal is sent from a line, a relay 14 is excited to insert a primary coil of a transformer 1 in series between a connection terminal L1 and a telephone set side terminal R, a chock coil 17 is inserted between a connection terminal L2 and the telephone set side terminal R and resistor 5 and a capacitor 6 are connected in parallel between the telephone set side terminals T and R. In the case of sending a data signal, a switch 12 is depressed to insert a primary coil of the transformer 1 between the connection terminal L1 and the telephone set side terminal R, the choke coil 17 is inserted between a connection terminal L2 and the telephone set side terminal R and the resistor 5 and the capacitor 6 are connected in parallel between the telephone set side terminals T and R.