When an outer circumference of a clamping portion 25 and an outer circumference of a circular outer circumference 26 are restrained by a die 6 while inserting a non-circular punch 5 into a circular hole 28, paddings 27, 27 are pressed by a semi-circular inner circumference of the die 6 to thereby be forced into interior of the circular hole 28. The volume of the paddings 27, 27 is set to be substantially the same as a volume obtained by subtracting the volume of a non-circular hole 23 in an extension shaft 2 from the volume of the circular hole 28. The circular hole 28 is deformed, and the paddings 27, 27 forced into the circular hole 28 abut with inclined planes 52, 52 of the punch 5, whereby the non-circular hole 23 is formed in the extension shaft 2.