Abstract: A method of connecting metal materials to each other, wherein a pin fitted to the tip of a metal bar-like rotating tool (10) is inserted between the end pan of a metal member (I) and the end part of a metal member (1?), and moved, while rotating, along the longitudinal direction of these end parts. By this frictional heat is generated between the metal members (1) and (1?) and the rotating tool (10), and the metal member (1) is connected to the metal member (1?). The rotating tool (10) is formed of a wide shoulder (12) and a thin pin (11) formed at the tip thereof and inserted between the end parts of the metal members. The pin (11) is a right circular cylindrical pin. The side face of the pin (11) is formed in a smooth curved surface, and a thread groove is not formed therein.

Abstract: A pin formed at front end of a rotary tool 10 made of a metal in a rod shape is inserted between an edge of a metallic member 1 and an edge of a metallic member 1?, which inserted pin is moved along the longitudinal direction of the edges while rotating the pin, thereby generating friction heat between each of the metallic members 1 and 1? and the rotary tool 10 to weld the metallic member 1 with the metallic member 1? together. The rotary tool 10 is structured by a wide shoulder 12 and a thin pin 11 formed at the front end of the shoulder 12 and being inserted between the edges of the metallic members. Here, the pin 11 is in a right-cylindrical shape, being formed into smooth curved surface on the side thereof, and having no thread groove thereon.