DATA TRANSMITTING METHOD

PROBLEM TO BE SOLVED: To automatically set a gain according to the attenuation in a line by detecting the size of a reference frequency signal fetched from a band-pass filter and fixing the size of the transmission output of its own station based on the size of a detected value. SOLUTION: The frequency f2 of the output of the second modulation/ demodulation circuit 2b of an opposite station and the frequency f3 of the output of a reference signal device 7 are added by an adder 8 to be the signal of f2+f3 to be outputted to a line 6 through a second hybrid coil 3b. Among the frequency f2+f3 received by its own station, only the signal of f3 is fetched by a band-pass filter 9 and the size of this reference signal is detected by a level detector 10. The gain of a first amplifier 4a of its own station is set by the detected value. When the detected value is smaller than an expected value, the line attenuation is large and the gain is set to be large. When the detected value is larger than the expected value, the gain is set to be small.