Abstract: A light beam scanner which is adapted to reflect optical beams by a rotary polygonal mirror provided adjacent a free end portion of a rotary shaft which is supported rotatably by a bearing, thereby performing scanning, the rotary polygonal mirror is covered with a protective cover, the protective cover being provided with a gas-inlet opening capable of introducing gas into the inside of the protective cover and a gas-outlet opening with the rotary shaft loosely-fitted therein, the gas-outlet opening permitting a discharge of gas from the inside of the protection cover. Accordingly, any deposition of oil mist which may leak from the bearing on the rotary polygonal mirror can be prevented by producing a gas-flow stream which travels from the gas-inlet opening to the gas-outlet opening.