A light source unit mounting structure includes a light source unit, including a reflector for reflecting light of a light source on a concave reflecting surface to give a directivity; and a lamp holder, on which the light source unit is mounted by securing an opening edge part of a light emission opening of the reflector to the lamp holder. An annular flange portion around the light emission opening of the reflector mounted on a bearing surface provided at the lamp holder is elastically pressed by two pressers fastened by screws to the lamp holder, so that the flange portion is elastically pressed and retained in a thickness direction between the pressers and the bearing surface. An outer circumferential end face of the flange portion is pressed against a receiving face having a step shape provided outside of the bearing surface of the lamp holder with an outside surface of the reflector elastically pressed radially by a lateral pressure applying member fastened by a screw to the lamp holder, so that the opening edge part of the light emission opening of the reflector is elastically pressed and retained radially between the lateral pressure applying member and the receiving face.