PROBLEM TO BE SOLVED: To process a twisting shape accurately and stably even if a twisting angle is large.SOLUTION: A recessed part 53 is formed by pressing each plane part 51 in a metal tube 5 toward a tube side by a roll pair 4a for forming a recessed part, when a metal tube 5 is transported in an order of a roll pair 4a for recessed part formation, a reference roll pair 4b, and a roll pair 4c for twisting, and a right edge or a left edge of the each plane part 51 is brought into contact with the plane from a substantially vertical direction by the reference roll pair 4b, the right edge or the left edge of each plane part 51 in the metal tube 5 is pressed by the roll pair 4c for twisting, which is provided by being rotated from a contact surface of the reference roll pair 4b to a circumferential direction outside the tube according to a desired twisting angle to a pitch of a pipe axis direction from the reference roll pair 4b.
