PURPOSE: To suppress an eddy current in a metal pipe which is caused by harmonic flux by a method wherein the outer circumference of a metal pipe is pressed by a roller having a blade-shaped tip to form a recessed trench by plastic deformation.

CONSTITUTION: A roller 21 having a blade-shaped tip is held by a roller holder 23 and pressed against the outer circumference of a metal pipe 3e and, in accordance with the rotation of the metal pipe 3e, a recessed trench 25 having a required depth is formed by plastic deformation. The roller holder 23 is horizontally transferred in an axial direction in accordance with the rotation of a spindle 26 to form the spiral recessed trench 25 in the outer circumference of the metal pipe 3e. The metal pipe 3e is coupled with the outer circumference of the magnet of a rotor and, when the rotor is placed inside a stator and driven as a motor, an eddy current is induced in the metal pipe by harmonic flux. However, in the recessed trench 25 part, especially in the thin parts whose thickness is T4, it is difficult to make the eddy current flow. As a result, the loops of the eddy currents are subdivided.