PURPOSE: To suppress the fluctuation of the level of a transmission signal by providing a third coil of same number of coils as that of the coil at a line side at a line transformer, terminating the coil by the same impedance as line termination impedance, and controlling the level of the transmission signal corresponding to the detection result of the power consumption of the coil.

CONSTITUTION: The third coil of the line transformer 3 is connected to a power detection circuit 4, and the output of the circuit 4 is set as the input on one side of a differential amplifier 5, and a reference voltage Vref is supplied to the input on the other side. The output of the differential amplifier 5 goes to the control signal of a gain variable amplifier 1, and the number of winding of the third coil is conformed to that of the coil at the line side of the line transformer 3, and it is terminated by the same impedance as the line termination impedance, and a line driver circuit 2 is a power source. In such a case, the power level of the transmission signal always coincides with the power detected at the power detection circuit 4. Thereby, the fluctuation of the level of the transmission signal due to the fluctuation of the source voltage of the line driver circuit 2 and temperature fluctuation can be suppressed.