A focus detecting device for detecting the focus adjusted state of an imaging optical system with respect to an object body comprises first and second photoelectric element arrays each including a plurality of photoelectric elements arranged in one direction, a focus detecting optical system for forming first and second optical images of the body on or near the first and second arrays by first and second light beams passed through different first and second areas, respectively, of the exit pupil of the imaging optical system, the first array producing a series of first output signals having a distribution pattern associated with the illumination intensity distribution pattern of the first image, the second array producing a series of second output signals having a distribution pattern associated with the illumination intensity distribution pattern of the second image, focus detection apparatus for producing a focus detection signal representative of the focus adjusted state of the imaging optical system, on the basis of the first and second output signals, and vignetting detection apparatus for detecting, on the basis of the first and second output signals, the vignetting influence of the first and second output signals by the vignetting of the first and second images caused by the imaging optical system.