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. A light amount distribution detecting device determines the decrease in light amount on the plane of photosensor elements of the photoelectric converting device as light amount information relating to the position on the plane of photosensor elements, encountered when the phototaking system and the focus detecting system are combined, based on the information from the phototaking optical information device and from the focus detecting optical information device. A focus detection calculation device determines the defocus amount of the current object image plane relative to the reference plane based on the light amount distribution information.