To cope with such a condition that lumber becomes dry and thin by making a joint metal with secured necessary strength as small as possible and preventing the joint metal from sinking into the lumber to be joined.

In the joint metal 15, an abutting plate 7, thinner than a joint base material 1, for preventing the sinking of the joint metal 15 is formed on the outer periphery of the L-shaped material 1 composed of one piece (a horizontal member) 2 and the other piece (a vertical member) 3, which have prescribed joining strength. A screw rod formed separately is screwed and caulked into a cylindrical rising part of a seat disk in order to be formed into a washer faced bolt. The screw rods of the bolts are made to penetrate the lumber to be joined, protruded from the through holes 4 and 5 of the pieces 2 and 3, fastened and fixed by nuts.