PURPOSE: To easily form longitudinal grooves in the desired positions of a metallic pipe by pressing dies onto the metallic pipe by means of pressing means, and forming grooves in axial direction between the dies and a mandrel.

CONSTITUTION: Split dies 4 are opened outward by moving wedges 7 rightward by means of hydraulic cylinders or the like, and a pipe 1 is inserted into the dies 4. At this time, a mandrel 2 fixed at one end is inserted into the pipe 1, and is so adjusted that its creset parts 2a oppose diametrically to the grooves of the dies 4. The wedges 7 are then pushed in leftward to approach the dies 4 slightly inward in a diametral direction, thereafter, a driving ring 10 is rotated. If the abutment points of the rollers 6 with backers 5 are moved from the position of 5a to 5c and further from the position of 5c to 5b, the leading end parts of the dies 4 give impact to the pipe 1, thereby pressing the pipe 1 to the mandrel 2. Since the four dies 4 press the pipe 1 simultaneously at this time, the pipe 1 is formed with longitudinal grooves over the entire circumference thereof.