To provide a laser printer having an illumination system capable of holding brightness excellent in uniformity.

A laser printer 10 consists of a laser diode array 11, an illumination optical system 20 crossing the array, a laser lenslets array 24, an optical modulator array 40 and a fly-eye integrator 23 illuminating an optical modulator with sufficiently uniform light. The modulator array 40 is uniformly illuminated by a laser diode emitter 12 composed of a laser diode array 11 to resolve light into image elements and, next, an image is formed on a medium surface 60 from the image elements and spots of desired patterns are formed.