PURPOSE: To estimate the amplitude characteristic of a telephone line and to set an equalizing circuit of an MODEM automatically, by detecting the spectrum of a reception signal, in a data transmission using the telephone line.

CONSTITUTION: The characteristic of an equalizing circuit 18 is set to a curve 14 before the reception of a training signal prior to data transmission. The reception signal inputted to an input terminal 23 is outputted to an output terminal 24 through an equalization circuit 18 and a demodulation circuit 19 and also inputted to a spectrum detection circuit 20, which detects the slope of the amplitude characteristic of the telephone line and to control an equalizing characteristic selection circuit 21, allowing to select one of equalization curves 14∼17 as the characteristic of the equalization circuit 18. The equalization curve once set is not changed until the reception carrier is interrupted with a timer 22.