PURPOSE: To increase or reduce the diameter of a pipe end by the frictional heat of a die which has a recessed part for diameter reduction or conic projection part for diameter increase, by pressing a pipe against the die and rotating the pipe.

CONSTITUTION: The die 1 which has the semicylindrical recessed part 2 having a specifying projection part 3 internally is made of a material with superior heat resistance and wear resistance. The metallic pipe 4 to be worked is clamped with a clamp 5 and set in the recessed part 2 of the die 1 and while the die 1 and pipe 4 are rotated circumferentially and relatively, the pipe 4 is pressed strongly against the recessed part of the die 1. Then, the tip part of the pipe 4 is softened by the frictional heat between the both and the opening end 4a of the pipe 4 is reduced in diameter along the recessed part 2 of the die until it is specified by the specifying projection part 3. When the opening end 4a of the pipe 4 is increased in diameter, a die having a conic projection tip is used to increase the diameter of the opening end 4a easily.