A focus condition detecting system, comprising an optical means for forming at least one pair of images of light from beams of light from at least one pair of different areas of an exit pupil of a cata-dioptric lens, at least one pair of light receiving means for receiving thereon the images of light from said optical means, at least one focus condition detecting means for detecting a relative displacement between the images of light depending upon outputs of said light receiving means to detect a focus condition of said cata-dioptric lens, a judging means for judging, depending upon data read from said cata-dioptric lens regarding outer and inner radii of the exit pupil of said cata-dioptric lens, whether or not optically accurate detection of a focus condition is possible from a combination of said optical means and an optical system of said cata-dioptric lens, and means for rendering said focus condition detecting means operative when said judging means judges that such optically accurate detection of a focus condition is possible.