An optical scanning device has a housing in which a duct surrounds a space from the deflector and its circumference to an outlet in insulation from the optical system, and guides gas from an inlet to the space. A positioning member is shaped as a pillar that protrudes from the floor of the duct towards the deflector and has a tip to contact with the deflector to locate the rotation axis of the polygon mirror at a position relative to the housing. The positioning member further limits a clearance between an outer surface of the deflector and the floor to secure therein a flow path of the gas.