In a link rod manufacturing method, a bracket material (14) having a ring portion and a mounting projection (15) is made by extrusion molding a material at the length corresponding to a plurality of brackets. On a forward end of the mounting projection (15), an engaging projection (17) with upper and lower steps (7a, 7a) is formed. An arm (2) is made by extrusion molding and regular-size cutting a material in a substantially I-shaped configuration which has a pair of first and second walls (20, 21) parallel extending on upper and lower sides and a third wall (22) crossing at a right angle and connecting these walls. A notch (23) is formed on an end of the third wall (22). When the engaging projection (17) is engaged with the notch (23), each of ends of the first and second walls (20, 21) on the side of ends 2a of the arm 2 is fitted onto the step (7a, 7a) and butted against a vertical wall (17b). Surfaces in the vicinity of butted portions (6) formed in this way forms a plane tool traveling surface and are supported on the insides thereof by the engaging projection (17) to prevent deformation. Therefore, the friction stir welding is carried out along the butt portion (6).