Provided is an optical scanner capable of preventing a temperature rise in a polygon mirror and its surrounding, and contributing to the prevention of degradation in image quality. Two cells (an inlet and an outlet) among a plurality of cells of a strengthening rib structure disposed in an optical base are connected through a passageway so as to form a flow path of outside air (air), and a bottom surface of a drive board forming part of an outer wall of a containing chamber and supporting a polygon mirror is exposed to the flow path (passageway). Even if a motor and a motor circuit disposed in the drive board generates heat during rotation of the polygon mirror, the drive board is directly cooled by the air flowing through the flow path, so a temperature rise in the polygon mirror and its surroundings can be prevented. Thereby, degradation in scanning characteristics of a laser beam due to deformation of an enclosure resulting from a temperature difference during heat generation can be prevented.