To realize dustproof measures against dust infiltrating into an optical box.

In a scanner which possesses a rotary polygon mirror to deflect a light beam emitted from a light source unit and to perform scanning with it, a driving means to rotate and drive the rotary polygon mirror, an image forming means to form an image with the light beam on a photosensitive body by passing it through the rotary polygon mirror and the optical box to support the image forming means, the light source unit and the driving means and in which the optical box is hermetically sealed by a cover member, a tacky adhesive part to which adhesive is applied is provided at the entire surface of the peripheral part of the cover member, so that it is stuck by the tacky adhesive part to the optical box.