PURPOSE: To always realize satisfactory speech in spite of the state of communication, by controlling the amplification degree of a signal amplifier circuit by receiving and detecting a required investigation signal according to on-hook at an incoming side.

CONSTITUTION: By raising a handset 7, the on-hook switch 8 of a telephone set system TH is turned ON, and the polarity of a speech line 6 is inverted. The fact is detected by a polarity inversion detection circuit 10, and the investigation signal of a sound frequency band area is transmitted in a short period via a CPU 11 and a signal delivery circuit 12. The investigation signal is received by a device TH similar to the device at the incoming side, and is detected by a signal detection circuit 13. The CPU11 performs the automatic switching of the amplification degree of an amplifier circuit 9 at the optimum value corresponding to detected output. Therefore, it is possible to always perform the satisfactory speech in spite of the state of the communication.