Half shell 10 has a bowl-like shape, and is composed of a thermoplastic resin composition. The half shell 10 has a top part 11 and a side part 12. The thickness Tt of the top part 11 is less than the thickness Ts of the side part 12. The difference (Ts-Tt) between both thicknesses is 0.02 mm or greater and 0.30 mm or less. Two half shells 10 and a core covered by the half shells 10 are placed into a mold comprising an upper portion and a lower portion both of which having a semi hemispherical cavity. Upon clamping, the thermoplastic resin composition is compressed while being heated, resulting in outflow of the excess thermoplastic resin composition from the cavity. A cover is formed having the nominal thickness of 0.3 mm or greater and 1.0 mm or less with remained thermoplastic resin composition.