Abstract: A self-threading screw comprises a screw main body of metal having a small heead portion in the proximal end of a threaded shank and a head of synthetic resin material fitted onto the small head portion. The synthetic resin head has a polygonal socket wrench engaging means formed on the circumferential peripheral thereof. The small head portion has a rotary tool engaging means so that a rotary tool can engage with the engaging means of the small head portion when the synthetic resin head is cut open. The synthetic resin head has an annular seal face formed in the bottom thereof while a stopper porton is provided having a greater diameter than that of the shank portion and extending downwardly of the seal face. Additionally, an annular packing is mounted beneath the seal face so that the stopper face is located between the seal face and the bottom face of the packing.