The time interval/amplitude level difference between sequential peaks in waveforms of first reproducing signals from a recording medium is detected and stored in a storage. The time interval/amplitude level difference between peaks in a waveform of a second reproducing signal from the recording medium is detected and compared with the time interval/amplitude level difference stored in the storage to thereby perform peak detection in the waveforms thereof. An original bit train of the first reproducing signal corresponding to a maximum-likelihood waveform selected among the waveforms of the first reproducing signals is given as a standardized signal. Data are provided to represent the time interval/amplitude level difference between the sequential peaks in the waveforms of the reproducing signals in a range of electromagnetic signal interference on the recording medium.