A rectangular plate optical reflecting mirror is provided in an optical scanner. The optical reflecting mirror reflects an optical beam onto a surface to be scanned. The optical reflecting mirror includes a center portion and end portions in a longitudinal direction. The center portion has a greater thickness than that of the end portions. Accordingly, rigidity of the optical reflecting mirror can be increased, and its characteristic frequency can be set at a high value. Thus, resonance of the optical reflecting mirror can be prevented from occurring as a result of vibration of a housing which vibrates with a plurality of vibration modes.