A headlamp in which light source units are tilted about predetermined tilting center axes by means of an aiming mechanism interposed between a lamp body and the light source units. The aiming mechanism includes a tilting fulcrum (a ball and socket joint), aiming screws which are rotatably supported on the lamp body, and nut members which thread fit on the screws and mounted on the light source units movably in forward and backward in response to the rotations of the screws, so as to tilt the light source units about the tilting center axes. A leaf spring slide portion brought into elastic engagement with a guide groove is made of a polyacetal resin which is superior in resistance to wear. A nut member main body having a screw thread fitting portion is made of a nylon resin which is superior in resistance to heat. There is no permanent set on the slide portion and a slide contact surface does not wear, and no looseness is generated in the screw thread fitting portion of the nut member main body. An accurate optical axis adjustment can be ensured.