PURPOSE: To constitute an equalizer by an IIR type filter with less hardware quantity by using an output signal of a transmission line and an error signal between the output signal and an output signal of a code discriminating circuit so as to estimate a polynomial of a numerator and a denominator of the transmission line.

CONSTITUTION: Let a transfer function of the transmission line 1 be B(Z)/A(Z) of the transmission line 1 and a signal yk is inputted to an estimate model 2 of the function A(Z). Further, an output signal of a code discrimination circuit 3 is inputted to an estimate model 4 of the function B(Z) and its a sum signal of its output signal and the output signal of the model 2 is used as an input signal to the code discrimination circuit 3. Further, a difference signal between the input signal to the estimate model 2 and the output signal of the code discrimination circuit 3 is used to control the estimate models 2 and 4. Thus, the polynomials of the denominator and numerator of the line 1 are estimated and the output signal of the line 1 is used as the input signal to an equalizer 5.