PURPOSE: To prevent projection due to piping by fitting an exterior wall board having a recessed sectional shape to an exterior wall in parallel with exterior wall panels, embedding piping in the recessed groove of the exterior wall board, and covering the recessed groove with a cover board.

CONSTITUTION: A recessed groove 11 is axially porvided on the front side of an exterior wall board 10 whose thickness is smaller than that of exterior wall panel 12, and at the same time a through-hole 13 communicating with the back side is provided at the upper part in the recessed groove 11. Next, the exterior wall board 10 is fitted to an exterior wall in parallel with the exterior wall panels 12. Next, at the recessed groove 11 of the exterior wall board 10, a drainage pipe 14 is arranged by means of an arch shaped metal fitting 15, and the upper part of the pipe 14 is introduced to the inside of a room through the through-hole 13. In addition, a cover board 16 is fitted to the front side surface of the exterior wall board 10 by means of metal fitting 18, and calking materials 20 are filled into the recessed joint 19 at both side edges of the cover board 16.