An optical scanner includes a light source modulated based on image data, an optical deflection and scanning part deflecting a light beam emitted from the light source, and a scanning and imaging optical system condensing the deflected light beam toward a scanning surface so as to form a light spot optically scanning the scanning surface. The effective scanning region of the scanning surface is divided into a plurality of regions according to a scanning line curving characteristic. Suitable image data for optically scanning the divided regions are selected from image data of a plurality of image lines every time the light spot optically scans the effective scanning region, so that the image data of each of the image lines is written with scanning line curving being corrected.