PURPOSE: To obtain a specified bearing strength on a regular basis and eliminate the need for machining a basic material at a structural steel fabrication shop which forces to increase a friction coefficient on a joint area.

CONSTITUTION: A joint auxiliary member 9 which forms a blade-shaped projected part 8 on one side of a plate-like body 7 which has at least both strength and hardness identical to a basic material 1 and a joint fitting 2 and a bolt hole 6 in the central part, and what is more, a clamping force as large as that of one strength bolt in scope, is fitted into a counter boring recessed part 11 formed around a bolt hole 4 on a joint area of a joint fitting to be connected to the basic material in such a fashion that a projected part on one side may be projected from the joint area. Clamping the basic material and the joint fitting by the high strength bolt forces the projected part to bite into the basic material definitely.