PURPOSE: To simply form drainage holes in a concrete retaining wall by a method in which an aluminum-covered steel wire is buried at the same time as concrete is placed, the aluminum cover layer is leached, and then the steel core portion is drawn out.

CONSTITUTION: In setting a formwork for a retaining wall 20, an aluminum- covered steel wire 10 is placed and fixed by an adequate method and then concrete wall is constructed. Whereupon, a high coagulation heat is generated during the curing period of the concrete, and the aluminum layer 2 of the aluminum- covered steel wire 10 is eroded by the heat and the alkali of the concrete and the steel wire 1 is loosened. The steel wire 1 is drawn out to form a drainage hole.