A focus detection device includes: a focus detection unit that includes a pair of light-receiving element arrays and lens systems via which a pair of focus detection light fluxes are each guided to one of the pair of light-receiving element arrays, and detects an image shift amount based upon output signals output from the pair of light-receiving element arrays; an acquisition unit that obtains a plurality of sets of opening data pertaining to sizes and positions of a plurality of openings; a function value calculation unit that calculates, based upon the plurality of sets of opening data, a function value pertaining to a gravitational center distance between gravitational centers of a pair of overlapping areas where a pair of specific focus detection pupils overlap an exit pupil; and a conversion unit that converts the image shift amount to a defocus amount based upon the function value.