PURPOSE: To reduce the fluctuation of the inner and outer diameters of a linear pipe having shifted depth and core, by manufacturing such that the thin and thick sides will come repectively to the inside and the outside of the curvature.

CONSTITUTION: A linear hollow pipe 7 having the depths a1 and b1 (a1>b1) corresponding respectively with the depths of outer and inner circumferences is selected. Then said pipe 7 is heated to the required temperature and the thin side is pressed against a rod zig 8 to be bent and formed into a hollow bent tube 9. Consequently the accuracy of the inner and outer diameters of said tube, the waving of the central diameter of the inner tube and the waving in Z direction of the pipe can be improved.