PURPOSE: To obtain a sharp corner part at the circumferential periphery without extremely increasing the internal pressure by forming by bulging a projecting part at a proper place of a tube, pressing the top face of the projecting part by a pressing die and forming a flat plane sharp in the corner part of the circumferential periphery.

CONSTITUTION: When the hydraulic pressure is applied from one side (arrow mark direction) inside a pipe 3 after the pipe 3 is sealed hermetically inside a bulge die 1, the pipe 3 is apt to be bulged inside the die 1, but bulging is obstructed by the hermetically sealed die 1, so the mass of the pipe is centralized to a part of the die 1b and it is bulged, the projecting part 4 along the die part 1b is formed. Next, when the projecting part 4 is pressed by a pressing die by operating a hydraulic cylinder, the bulge forming hydraulic pressure is made as a reaction force receiver, the top face 4a is made in a flat projecting part 4, the corner part of circumferential periphery 4b is made sharp and the total top face 4a of the projecting part 4 can be made flat at the same time.