To obtain an optical scanner capable of facilitating the adjustment of scanning pitch and converting the density of printing dots into a specified value with respect to a laser beam printer, etc., using plural-beam scanning.

This scanner is provided with a focal distance variable lens 2 arranged between a light beam generating element 1 and a rotary polygon mirror, and a detector 8 detecting the distances of plural beams, which are deflected to perform scanning, in a scanning perpendicular direction. The focal distance of the lens 2 is varied based on a signal from the detector 8 so that a distance between scanning beam spots may be a specified value. By varying the focal distance of the lens 2 in accordance with a signal from the outside and also varying the revolving speed of the mirror 4 and the modulation frequency of plural beams, the density of the printing dots is changed to the specified one.