Abstract: Disclosed is a focus detecting device for use in a camera comprising a phototaking optical system for forming an image of the object on a reference plane. A focus detecting optical system separates light passing through the phototaking system from at least a pair of light beams passing through spatially different areas on a first predetermined plane axially spaced by a first distance from the reference plane, and thereby forming images of the object. A photoelectric converting device generates object image signals corresponding to the intensity distribution of the object images formed by the focus detecting optical system. A phototaking optical information device generates information on the shape of exit pupil at fully-open diaphragm of the phototaking system and on a second distance from the reference plane to the exit pupil. A focus detecting optical information device generates information on the areas on the first predetermined plane and on the first distance.