To miniaturize a focus detecting optical system by receiving a luminous flux from a subject, deflected by a deflection member, in the focus detecting region of the peripheral part of a photographic picture frame, so as to detect the focusing state of an optical system.

The reflection member is installed in a position 90 on the photographic optical system side of an expected focal plane 9, between a photographic optical system and its expected focal plane 9. The luminous flux from the object passing through outside an optical axis on the photographic subject system side is deflected to the side of the optical axis 7 at the position 90 nearer to the photographic optical system side than the expected focal plane 9, by the deflection member. A part of the deflected luminous flux from the subject is made incident on the focus detecting region of the peripheral part of the picture frame set in the position away from the intersection between the expected focal plane 9 and the optical axis 7. Therefore, a photodetector 51 corresponding to the focus detecting region of the center of the picture frame and photodetectors 52 and 53 corresponding to the focus detecting regions of the peripheral part of the picture frame can be formed on an image sensor chip. In other words, the dimension of a visual field mask 10 can be made small and an aperture in the vicinity of the expected focal 9 of a focus detecting module can be made small.