To obtain a long-sized external angle member which can be easily and surely attached to a building frame without nailing work from the surface side, and it construction method.

To the rear surface of a long-sized external angle member 10 having a length in up-down direction larger than the height of an exterior finish member 20 in up-down direction, in attaching board member 14 having a narrow width to be used for attaching to a building frame side is attached in one united body in such a manner that the side portion is projected out from the side edge of the long-sized external angle member 10. The long-sized external angle member 10 is arranged at the external angle member at the building frame side and fixed to the building frame side by nailing the attaching board member 14. After attaching a joiner 4 as required, the exterior finish member 20 is attached in multi-stage adjacent to the side edge of the joiner in up-down direction.