To prevent inner and outer panels from being recessed or cracked due to aging or chemical reaction of a foaming agent by arranging, on the inner surface of a panel constituting a case body, with a member for preventing adhesion of a heat insulating material to the panel surface.

Inner and outer panels 13, 14 constituting side plates are arranged oppositely at an interval with an oil and fat 16 being applied to the facing surfaces and then a foaming agent is foamed. Consequently, a boundary layer by the oil and fat is formed between the foaming agent and the panels 13, 14 thus preventing adhesion of the foaming agent to the panels 13, 14. According to the structure, the panels 13, 14 are not recessed even if the foaming particles are degassed due to aging of the foaming agent.