PURPOSE: To make such a firm coupling state that is high in reliability securable, by pressing a member, having a fitting part where irregularities in triangular-tooth serration form, in an end inner circumferential surface of a steel pipe.

CONSTITUTION: A cotter 5 consisting of a flange 8 blocking an end face of a steel pipe 1 and a fitting part 6 to be pressed in an end inner circumferential surface of this steel pipe 1 is formed with S45C. Triangular-tooth serration irregularities 7 are formed on an outer circumferential surface of this fitting part 6 parallelly in the longitudinal direction over the whole circumference and then induction hardening takes place. The cotter 5 constituted like this is pressed in the end inner circumferential surface of the steel pipe 1 after applying a lubricant to the triangular-tooth serration irregularities 7 of the fitting part 6, and the flange 8 is made to adhere to the end face of the steel pipe 1, blocking up the end face. At this time, the plastic binding power made by what a tip part of the triangular-tooth serration irregularities 7 is bitten into an inner circumferential surface 4 of the steel pipe 1 is added thereto.