An upper die and a lower die are provided to form a cavity. The upper die and the lower die have respectively slots made through them in parallel with each other, and the slots are connected to the cavity. Also, the slots have pins reciprocatably inserted through them. The pins are reciprocated such that they are advanced to greatly project their ends into the cavity, and that they are returned to allow their head portions to remain slightly projected into the cavity. The pins respectively comprise a body portion in a bar-shaped configuration, a head portion connected to the end surface of this body portion. The end surface of the body portion is furnished with the substantially same radius of curvature as the bottom surface of the cavity. Also, the body portion is shaped in the surface thereof so as to have a circular configuration as viewed from the center of the cavity.