PURPOSE: To detect focus by forming an image through the objective lens of a camera on a difraction grating rotating around the optical axis and forming the transmitted or reflected image through/on the diffraction grating on a self-scanning type solid-state pickup element with two-dimensional array.

CONSTITUTION: The image of a subject through an objective lens 1 is formed on the surface of a transmission type diffraction grating 2 and the grating 2 is fixed on a frame 6 and rotated around the optical axis by a motor 8. A two-dimensionally arrayed self-scanning type solid-state pickup element 3 is arrnged on the back of the grating 2 and the image on th grating 2 is formed on the surface of the element 3 through an image formation lens 4. The output of the element 3 is connected to a detector 5 to output a time-series signal obtained by scanning the element 3. The rotating speed of the grating 2 is set up slower than the scanning speed of the element 3 and, when 2 time-series signals obtained by scanning the element 3 with time interval coincide with each other, the focusing time can be dtected.