To solve the following problem; in an optical scanner (galvanometer mirror) which performs scanning with light emitted from a light source by rotationally vibrating a reflecting mirror, it is necessary to vibrate the mirror with a resonance frequency (natural frequency) in order to reduce power consumption and enlarge a scan width by suppressing an influence of heat generation, but the resonance frequency varies due to the influences of dimensional errors of a movable plate part and a torsion bar, the quality of the material, heat, and the like and the amplitude varies, therefore, fluctuation of the scan width arises and quality of printed characters by a printer is degraded.

The optical scanner is so structured that the movable plate part is supported by two or more supporting beam parts 3a to 3d arranged near the rotation axis of the movable plate 2 on which the reflecting mirror 1 is formed, and is provided with a mechanism capable of controlling axial force applied to the supporting beam parts by an actuator or a screw such as a laminated type piezo-electric element. With such a structure, the natural frequency increases/decreases approximately in proportion to the axial force applied to the supporting beam parts, therefore, the natural frequency can be adjusted to a predetermined value by controlling the axial force.