There is a focus detection device known in the related art that adopts the split-pupil phase detection method (see patent literature 1). This focus detection device generates a pair of image signals in correspondence to a pair of images formed with a pair of focus detection light fluxes passing through the exit pupil of an optical system. A correlation operation of the known art is executed by shifting the pair of image signals relative to each other, so as to calculate a correlation value representing a degree of coincidence between a pair of image signal strings resulting from the relative shift. Based upon the correlation value, a shift amount at which the highest agree of coincidence is achieved for the patterns expressed in the pair of image signal strings resulting from the relative shift, is detected as a relative image shift amount representing the extent of relative image shift manifested by the pair of subject images. In addition, the focusing condition of the optical system is detected in correspondence to the image shift amount. The focusing condition of the optical system is indicated by the difference between a predetermined focal plane and a detected image plane, i.e., by a defocus amount.