PURPOSE: To remove noise by simplifying information processing of a focus detecting circuit by making use of the main space frequency information of a subject.

CONSTITUTION: A subject is reflected by a mirror 2 and is imaged on a screen 3 by a taking lens 1. If the screen 3 is a perfect diffusion plate, the Fourier information of the subject is imaged on an information imaging plane 5 by a Fourier transform lens 4. If a plurality of photodetectors S0...Sn are disposed on said imaging plane 5 in the radial direction from the optical axis, the output with respect to space frequencies are obtained. Said lens 1 is moved from the front focus position up to the rear focus position and the outputs of the photodetectors S0...Sn are mutually compared by multiplying the respective outputs by the specific coefficients, whereby the main space frequency of the subject may be determined. The output signals of picture elements are emitted by scanning of a photosensor array 6 and are sampled through a scanning control circuit, whereby they are successively compared and are arithmetically processed.