PURPOSE: To prevent the generation of wrinkles by controllably arranging at least two mandrels relatively in the longitudinal direction of a tube during bending.

CONSTITUTION: A DD tube 6 is provided with two single D tubes 7, 8, which are separated by a common compartment wall 9 each other. The tail end part of the tube is tightened by a gripper 20. The gripper 20 is rotated around the axis coincident with the longitudinal direction of the tube. In order to form S bending, the gripper 20 and two bending mandrels 12, 13 including pulling bars 16, 17 and a rocking arm 18 are rotated 180° around the axis in the longitudinal direction of the tube. After rotation, the D tube 8 having the bending mandrel 12 in the inside is positioned facing a radial bending block, and the D tube 7 having the bending mandrel 13 in the inside is positioned facing a slide shoe 21. Thereafter, bending processes are continued, and whereby the bending mandrels are arranged in the correct position again.