A nonlinear compensation A/D converting method and a circuit for compensating asymmetry of a reproduction signal waveform, so as not to deteriorate the performance of a discriminator even if there is a nonlinearity such that the amplitudes of the signal differ in dependence on the polarities of the waveforms like a reproduction waveform of an MR head are provided. Scales 1+ and 1- of an A/D converter are set to values which differ in dependence on the positive and negative polarities of the input signal, so that a nonlinear compensation which is suitable for a high accurate and high speed operation can be realized. Even an MR head in which the asymmetry between the upper and lower waveforms of the reproduction waveform is large can be used. A yield of the head can be improved.