void space 4 the tubular component 2 is formed to be tubular from a pipe or the like and has both open ends although the cross section of the void space 4 inside the tubular component 2 has a rectangular configuration in the present embodiment this should not be considered a limitation of the cross sectional configuration the void filling material 1 is retained by a holder for void filling material 6 specifically the holder for void filling material 6 in the first embodiment is formed into a rectangular plate like configuration provided with a through hole at the approximate center thereof the void filling material 1 is inserted into the through hole and retained such that the void filling material 1 protrudes from both surfaces of the holder for void filling material 6 the length b 1 shown in fig2 a of the long side of the holder for void filling material 6 is designed to be greater than the length a of the diagonal line of the void space 4 of the tubular component 2 b 1 gt a the size of the void filling material 1 is selected so as to allow the holder for void filling material 6 to be inserted into the void space 4 by elastically deforming the same as shown in fig1 even.