To apply a light source of lower power without deteriorating picture quality as a light source (LD) for emitting a monochromatic laser beam.

The color optical scanner is provided with a color printing mode for executing color printing and a monochromatic printing mode for executing monochromatic printing at a speed higher than the printing speed of the color printing mode. The optical efficiency of an optical component existing in a laser beam optical path from the light source up to an image surface is selected so that the optical efficiency in an optical path to be used for the monochromatic printing mode is larger than the optical efficiency in the other optical path. For instance, the value of the optical efficiency can be changed by the number of reflection mirrors existing in each optical path.