To reduce a deviation from a desired dimension by inserting polymer mandrels from both sides of a hollow metal body and bending while stretching the hollow metal body from both ends in the longitudinal direction to prevent collapse of the hollow metal body due to bending.

A hollow extruded work 20 is mounted between gripper assemblies 11, 12, mandrels 35, 36 of a solid polymer mandrel are pressed/inserted thereinto from opening ends 37, 38 of the extruded work 20 by using piston rods 31, 33. At this time, a lubricant is supplied by a fiber core adjacent to an outer wall 40 through opening parts of guides 25, 26. Jaws 13, 14 are fastened. The extruded work 20 is bent while stretching by hydraulic cylinder assemblies 27, 28 on guides 25, 26 in the longitudinal direction. Successively, the extruded work 20 is again stretched to remove residual stress. The jaws 13, 14 are released from the end parts 21, 22 of the extruded work 20, the mandrels 35, 36 are mechanically removed.