A focus detecting device comprises a first photoelectric conversion section for performing photoelectric conversion of a light beam emitted from a first pupil area of a image pickup optical unit, a second photoelectric conversion section for performing photoelectric conversion of a light beam emitted from a second pupil area different from the first pupil area, a light intercepting section having openings for allowing passage of a portion of light in the first pupil area and for allowing passage of a portion of light in the second pupil area, and a detecting section for detecting a focus condition of the image pickup optical unit on the basis of photoelectric conversion outputs of the first photoelectric conversion means and the second photoelectric conversion sections. For example, when the image pickup optical unit has a large defocus quantity, the light intercepting section is set in an optical path for focus detection.