A vehicular headlamp of a type having a lighting circuit unit mounted on an outer surface of a bottom wall of the lamp body in which failure of the seal between the lamp body and a front lens mounted thereon is prevented. A bottom wall extension portion is formed at a rear end portion of a bottom wall of the lamp body, extending rearward beyond a bulb replacement opening formed in a rear wall of the lamp body. This structure provides sufficient rigidity for the rear end portion of the bottom wall, which otherwise would have insufficient rigidity because of its location adjacent the bulb replace opening. Therefore, the bottom wall is protected from deflection deformation due to the weight of the lighting circuit unit. As a result, the front end opening of the lamp body is protected from deformation.