To easily and precisely form a groove in axial direction of a tube.
A mandrel is inserted into at least a section to be worked of a tube, and the tube and the mandrel are held so that they do not move relatively. The tube and a die are relatively moved in the axial direction so that the tube is inserted into the die. A plastic flow is caused in tube material by a protrusion part 7 protruded from the die 5 to form a groove in the axial direction of the tube on the surface of the tube. A working face 10 of a front edge of the protrusion part 7 is inclined to have a sweepback angle for a relative advancing direction as well as is formed to a projecting curved face in the relative advancing direction, the plastic flow of material is promoted in the center direction and peripheral direction of the tube.
IRIE TORU
MORIKAWA AKINOBU
