PURPOSE: To quicken and simplify water proofing work and uniformalize the working quality.

CONSTITUTION: A nail head cap set is fitted to around the nail head 11 of a construction nail 1 joining between a sheathing member 3 and a bed member 5, and watertight around the nail head. The nail head cap set is composed of a cap 2, covered over the nail head 11 to watertightly protect the nail head 11: and a cap rest 4, interposed between the outer surface of the sheathing member 3 and the nail head 11 to make around a nail hole port be watertight, and also to fix the cap 2 in a closely fitting condition.