PURPOSE: To prevent a signal from making a detour, and to increase the gain of an amplifier part by adding and offsetting the same signal which is brought a leakage signal in a two-wire/four-wire converting circuit to phase inversion.

CONSTITUTION: When a two-wire telephone line is brought to speech connection, DC loop information is sent out to a microprocessor 11. The processor 11 operates relays 17, 18 and inputs a 1kHz signal from a sine wave sending-out circuit 14 to fixed gain amplifying circuits 5, 6. Three 1kHz signals are amplified, respectively, and outputted to two-wire/four-wire converting circuits 3, 4 and gain variable amplifying circuits 7, 8. The 1kHz signals which is made a detour to a four-wire receiving side by the circuits 3, 4 are inputted to same phase adders 9, 10, respectively, added to a 1kHz signal being 180° out of phase inputted from the circuits 7, 8, passed through BPFs 12, 13 and received by the processor 11. The processor 11 adjusts the gain of the circuits 7, 8 so that these values become the minimum value being near '0', and release the relays 17, 18 after the adjustment. By this adjustment, a detour of a speech can be prevented to the minimum of the function of the circuits 8, 7.