To make a broken polygonal part pass through the socket hole of a binding tool while the bolt is easily manufactured by cold heading.

By pressurizing a rod-like base stock having a prescribed length in the longitudinal direction, a shaft part 1, a head part 3 which is continued to the shaft part 1 and a cylindrical projecting part 4" having a smaller diameter than that of the shaft part 1 which is twisted off by a specified tightening torque in the middle of the top surface of the head part 3 are formed by cold heading. Next, by forming the polygonal part 4 having a size which is not less than the outside diameter of the projecting part 4" by cold heading by pressurizing this projecting part 4" in the longitudinal direction, a twisting-off breaking part 5 is formed between this polygonal part 4 and the head part 3.