Abstract: A focus detecting device comprises an image forming optical system, a device for detecting the defocus amount with respect to a predetermined plane of a plurality of images, formed by the image forming optical system, of fields corresponding to a plurality of detection areas which are set beforehand, a device for classifying the plurality of detection areas into a plurality of groups on the basis of the defocus amount, a device for selecting one optimum group from the plurality of groups, and a device for determining one final defocus amount on the basis of the defocus amount corresponding to detection areas belonging to the optimum group.