To measure the nonlinear distortion of a magnetic disk even at a high recording density, a nonlinear transition shift and a partial erasure are separately measured.A periodic pattern containing dibits and isolated bits, and a periodic pattern consisting only of isolated bits, are written and read in synchronism with the rotations of the magnetic disk. The reproduced waveforms of the respective periodic patterns are frequency-analyzed in a state where the phases of the periodic patterns are in agreement, thereby to obtain odd-numbered order harmonic components. The partial erasure and the nonlinear transition shift are evaluated on the basis of the ratio between the odd-numbered order harmonic components.