PURPOSE: To provide a processing body which can be installed easily to the hollow of a hollow structure such as a vehicle pillar, and a secure installing structure of a foaming material.

CONSTITUTION: A processing body for foaming material installing which has four elastic pieces 2 of rectangular parallelepipeds having round outer forms, at the up and down, and left and right symmetric positions of a central part 1, is placed at a specific position on an inner pillar part 12 positioning the four elastic pieces 2 at the up and down, and the left and right parts of the hollow 16. When the inner pillar part 12 and the outer pillar part 14 are fixed by a spot welding 17, the four elastic pieces 2 are squeezed to the pillar inner walls to generate an elastic deformation, and the processing body is pressure-connected elastically and fixed to the hollow 16. Furthermore, by providing antiskid members 3 made by forming plural grooves having a saw-tooth form cross section parallel, to the elastic pieces 2 at the parts contacting to the inner surfaces of the hollow 16, a slippage of the processing body is prevented, and the processing body can be installed more securely in the hollow 16. Since the foaming material is formed by the heating from the external side, and a forming substance is filled in the hollow 16, a soundproof effect and a damping effect are given to the pillars.