PURPOSE: To provide chair parts which have a light weight, a high rigidity, and an excellent fatigue characteristic and prevent phenomena such as shrinking, warping, strain resulting from uneven quenching, curtail the molding hours, and reduce the molding cycle, because they are molded by hollow injection of a synthetic resin and hence they have a cavity in the thick section.

CONSTITUTION: At the same time when a molten resin is injected into the cavity of a metallic molding or after that, a high pressure gas is introduced to the molten resin in the metallic molding. The chair parts have a rib structure molded by the above way and also have a cavity at the root of a rib and/or at the thick section of the root of the rib.