To prevent the defective foaming of a foaming base and shut off a hollow chamber in a good manner.

A hollow chamber shut-off tool of a hollow structure is provided with a holder 20 set in a hollow chamber 7 of a hollow structure 1 and a foaming base 11 supported by the holder 20 and foamed by the external heating and formed into a foamed body to shut off the hollow chamber 7, and the holder 20 is set in the direction crossing orthgonally the longitudinal direction of the hollow chamber 7 and provided with a pair of supporting plates 21 supporting both side faces of the foaming base 11 and controlling the foaming direction of the foaming base 11. Foaming acceleration means 12 and 23 for increasing the heat transmission by the external heating and accelerating the foaming of the foaming base 11 are provided between the foaming base 11 and a pair of supporting plates 21.