For an input signal with a ringing superposed thereon, a ringing-generating filter generates an analogous ringing waveform from only a peak portion of the signal which precedes the ringing. A subtractor subtracts the analogous ringing waveform from the input signal to eliminate the ringing. The coefficient of the filter is determined by applying a calculation method similar to a polynomial division based on the complete pivoting Gaussian elimination to polynomials using a reference data expressing a peak waveform and a ringing waveform, and by using a least squares method for minimizing the square of the covariance so as to allow the presence of noise in the data. Furthermore, by a repetitive process on a plurality of the same datasets, the calculation accuracy of the coefficient is improved even under the condition that the ringing frequency is high and the number of samples in one cycle is small.