PURPOSE: To enable uniform heating of the bent tube part and straight tube part by performing bending work properly combining advancing and stopping of the tube and retreating and stopping of a heating device while keeping the ratio of relative speed to supplied amount of heat of the tube and heating device.

CONSTITUTION: A tube 1 is inserted and advanced and heat treated with a heating device H in stopped state. The tube 1 is stopped and heat treated while retreating the heating device H and the tube 1 is held by a bending arm 4. Then, the heating device H is stopped and the tube 1 is advanced and bending moment is given to the tube 1 and bent. The tube 1 is stopped and heat treated while retreating the heating device H. At the same time, the tube 1 is released from the bending arm 4. The heating device H is further retreated, and the tube 1 is advanced and heat treated. Finally, the heating device H is stopped and the tube 1 is advanced and heat treated. Relative speed (v) of the moving speed (w) of the tube 1 to the moving speed (vt) of the heating device H is always kept constant.