Detection of a difference in phase of an optical harmonic with the direction of a residual polarization by comparing the optical harmonic in phase with another optical harmonic generated in a uniformly polarized reference member arranged e.g. in an optical path other than in the recording/reproducing member. The former optical harmonic is generated by recording a signal on a ferroelectric recording/reproducing member as the residual polarization and irradiating this member with reproducing light flux. A conventional optical recording/reproducing technique is utilized, so that the utilizability of the optical recording/reproducing member as a multilayer medium is high. Therefore, the optical recording/reproducing layer is easily multipled, and a recording capacity is markedly increased.
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