PURPOSE: To prevent the occurrence of a trouble, such as leakage of a stock solution of a foaming agent through a communicating opening, by closing a foaming chamber partitioned by an inner surface wall and a communicating opening for communicating the foaming chamber with the outside with the plug member of a subframe body from the foaming chamber side and injecting the foaming agent in the foaming chamber to foam and cure the foaming agent.

CONSTITUTION: In a first process, a communicating opening 11 of a subframe body 1 is closed with a plug member 2 from the foaming chamber 10 side. In a second process, a given amount of a stock solution of a rigid polyurethane foam is injected through an injection port 12 of a subframe body 1 in the foaming chamber 10. In this case, a raw material having low viscosity is guided to an induction passage 14 and stored in a liquid reservoir part 13. By using a cover member 42 comprising a cover plate 20 and a curved pipe 41, the bottom of the cover plate 40 is coated with a mold release agent, and the cover member 42 is attached to the injection port 12 of the subframe body 1. When, with this state, a given time elapses, a stock solution stored in the liquid reservoir part 13 is gradually foamed and cured, and a foaming agent 15 formed of a rigid polyurethane foam is formed. The foaming chamber 10 is filled with the foaming agent 15.