To attenuate sound waves in a hollow chamber of a hollow panel and reduce the residual sounds generated by the reflection of sound waves without complicating a structure.

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, 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 supporting plate 21 supporting one side face of the foaming base 11 and controlling the foaming direction of the foaming base 11. A plurality of through-holes 22 are formed through the supporting plate 21 so that a part of the foamed body is so protruded on the outer side face of the supporting plate as to form a foamed protrusion when the foaming base 11 is foamed and formed into the foamed body.