An optical information processor according to the present invention comprises a spatial light modulator 1 illuminated by linearly polarized coherent light and a linear polarization detector element 2 for detecting light diffracted by the spatial light modulator 1, wherein a rotatory polarization element of the spatial light modulator 1 is arranged in such a manner that the direction of the major molecular axis 11 thereof at the incident light side substantially conforms to the linear polarization axis direction (Y-axis direction) of the coherent light and the linear polarization detector element 2 is arranged in such a manner that the detecting axis 21 thereof crosses substantially at right angles with the major polarization axis of the zero-order diffracted light 4 included in the diffracted light described above. As a result, it is possible to effectively reduce the zero-order diffracted light component which is not modulated by the spatial light modulator without increasing the entire length of the optical system so much and without requiring excessive electric circuit or processing time.