An outer casing for headlights and a headlight using the outer casing can include an outer lens having an opening portion and a casing having an opening portion. The opening portion of the outer lens can include a sealing projection and a water guard rib, and the opening portion of the casing can include an end surface and a sealing concave portion. The outer lens can be attached to the casing so that the sealing projection is inserted in the sealing concave portion while the water guard rib contacts the end surface. The outer lens can be casted by a simple molding tool such that a burr is prevented from occurring on the opening portion. Therefore, the headlight using the outer casing can incorporate various lamps with an airtight structure and an adequate mechanical strength even when the outer lens is formed in a thickness of less than 2.5 millimeters.