An auxiliary light projection apparatus used for focus detection has a laser beam generating device and a diffraction plate for diffracting a laser light from said laser beam generating device. The diffraction plate has micro lenses regularly arranged in vertical and horizontal directions. The curvature of each micro lens in the vertical direction is different from that in the horizontal direction. The pitch of arrangement of the micro lenses in the vertical direction is different from that in the horizontal direction. In addition, the height of the micro lenses is varied cyclically. The auxiliary light projection apparatus projects a diffraction light pattern of the laser light emergent from the diffraction plate to an object to be photographed, as a pattern used for focus detection.