An optical box for an optical scanner includes an optical housing and a lid. The optical housing has an opening and a side wall surrounding the opening. The side wall includes inward recessed parts, each of which has a protrusion formed on its outer side. The protrusion can engage with the lid. The lid can so engage with the optical housing as to cover the opening of the housing. The lid has a pair of dust guard walls, between which the whole side wall of the optical housing is sandwiched when the lid is in engagement with the housing.