PURPOSE: To simplify matching of optical axis very much, by utilizing cylindrical graded lenses for Fourier transform and by making their end faces into the Fourier transform face.

CONSTITUTION: The rear end face of cylindrical graded lens 11 for Fourier transform where the first fucus is placed on the front end face, to which the processing input image is incident, and the second focus is placed on the rear end face, and the front end face of cylindrical graded lens 11 for reverse Fourier transformation where the first focus is placed on the front end face and the second focus is placed on the rear end face are opposed to each other. These two lenses 11 and 11 are joined through space frequency region filter 16.