The invention provides a holding jig for supporting and fixing a solid material capable of being foamed by application of heat in a cavity of a hollow structure. The holding jig may be provided with a plate-like base body made smaller in diameter than the cavity of the hollow structure, a holding part that is provided in a predetermined width in the form of a ring on the periphery of one side of said base body and supports one side of the foamable material, a step part with predetermined height, which is projected in the form of a ring at the inner peripheral end of said holding part and supports the inner peripheral surface of the foamable material, and an attaching means with which the base body can be freely attached to or detached from the hollow structure. The holding part can be provided with a plurality of small holes that open from one side to the other side of the base body, preferably in a net or lattice pattern. The holding jig may have one or more extensions containing approximately U-shaped foaming control walls that assist in directing expansion of the foaming material towards corners of the hollow structure.