Long and short legs have heads and pointed tips, respectively. They are joined together near their tips by welding. An upper portion above the welded portion of the long leg is formed with a cut extending obliquely upwards toward the short leg so that a thin breakable portion is formed in the long leg between the end of the cut and the contact surface with the short leg. A washer for clamping the legs above the cut is provided. When the nail is driven, the legs will not spread until the head of the short leg abuts the washer. When the head abuts the washer, the long leg slides outwards and spreads apart from the short leg near the cut. The breakable portion will thus get broken and the nail will spread.