PURPOSE: To easily and costlessly obtain a plurality of geometrical shapes different from each other by providing an external cup-shaped part forming a hollow inside and having an opening and a cylindrical part extending from the opening to the substantially cup-shaped inside.

CONSTITUTION: A hollow body 1 is provided with an external cup-shaped part 2 forming a hollow inside 3 and having an opening 4 and a cylindrical part 5 extending from the opening 4 into the inside of the substantially cup-shaped part 2. The hollow body 1 and the cylindrical part 5 are integrally made of a plastic material. The external cup-shaped part 2 is substantially spherical and provided with a flat surface 6 oppositely arranged to the opening 4. The cylindrical part is contracted toward the inner end 7 of the opening, far from the opening 4. The circular cylindrical part 8 is fitted to the inner end 7 and extended to the opening 4. The circular cylindrical part 8 is ring-shaped and provided with a screw thread 9 at the outside.