An optical scanner can write a full-color image without occurrence of writing position differences even if a record density is switched. The optical scanner includes a black writing illuminant for optically writing an image at a plurality of record densities and a color writing illuminant for optically writing the image at a predetermined record density. The optical scanner adjusts a resist position for a full-color image with respect to the main and sub-scanning directions by switching a writing position of the black writing illuminant in accordance with a requested record density and then writes the full-color image at that writing position.