To heighten arranging accuracy of respective center positions of circular bored holes relative to wall plates when through holes are formed on a double-wall.

A first through hole H11, to which an opening 21 of a ventilation pipe holding tool (arranging element) S arranged in a space 4 is faced, is formed from outside of one wall plate 2 by a first circular drill hole using a hole saw (drill tool). A center position C22 of a second circular drill hole specified by a center position indicator E1 for indicating the center position C22 of the second through hole H12 to be formed on the other wall plate 3 passes through the first through hole H12 and the ventilation pipe holding tool S. A center position displaying through hole is formed on the other wall plate 3 by a long drill from the outside of one wall plate 2, and a center shaft of the hole saw is made to contact a through hole 51 from the outside of the other wall plate 3, so as to form a second through hole H12 having a specific positional relationship to the first through hole H11 on the other wall plate 3.