A method of detecting a focus condition of an imaging optical system such as a camera, a microscope, a high density optical recording and reproducing apparatus or the like is disclosed. The method comprises a step of selecting, in accordance with the state of density distribution of the optical image, between an image sharpness detection method for detecting in-focused condition by obtaining an evaluation value representing sharpness of images projected upon a photoelectric detecting means and an image lateral shift detection method for detecting in-focused condition by a positional relation between an image projected upon the photoelectric detecting means by light fluxes transmitted through a part of an exit pupil of an optical lens and an image projected upon the photoelectric detecting means by light fluxes transmitted through the other part of the exit pupil of the lens.