PURPOSE: To provide the two-wire/four-wire converter which corrects pseudo echo and does not remain any echo signal by measuring the echo signal flowing from a transmission circuit to a reception circuit and inputting these measured data to a digital filter.

CONSTITUTION: Before communication is started, a signal for measuring frequency characteristics is outputted from a frequency characteristic measuring instrument 9, and the frequency characteristics of the echo signal are measured. Next, a digital filter 8 sets the frequency characteristics of the filter based on the frequency characteristic data of the echo signal from the measuring instrument. Afterwards, when a data selector 10 is switched for inputting a four-wire transmitting signal to a D/A converter 6, one part of the four-wire transmitting signal flows from a hybrid circuit 1 through a converter 6 and an amplifier 4 to an amplifier 3 for reception. Then, it passes through the same passage as a received signal and appears at a four-wire reception output as the echo signal. Therefore, a four-wire transmitting signal is worked and inverted to the size corresponding to the amount of the echo signal by using the filter 8, and these signals are added to an output from an A/D converter 5 by an adder 7 and turned to a signal remaining a two-wire received signal.