To certainly restrict the foaming of a foamable base material in the longitudinal direction of the hollow chamber of a hollow structure by preventing the unexpected exansion or damage of a pair of foaming restricting support plates and enhancing the transmission of heat and to efficiently and certainly cut off the hollow chamber by accelerating foaming while preventing foaming inferiority in the direction crossing the longitudinal direction of the hollow chamber.

In an attaching structure for attaching a foamable base material 10 cutting off a hollow chamber by foaming the foamable base material by external heating to the hollow chamber of a hollow structure, a pair of foaming resticting support plates 20 restricting the foaming of the foamable base material 10 are arranged on both sides of the foamable base material 10 and supported on the support part 42 of the attaching member 40 fixed to the inner wall surfaces of the hollow structure. A pair of the foaming restricting support plates 20 are formed from rigid metal panels and mutually connected by connection means.