PURPOSE: To provide a high frequency bending method to a large diameter complicated pipe free from flaws on the inside surface of the back side.

CONSTITUTION: The complicated pipe 3 wherein an inner layer 2 molded of corrosion resistant Ni base alloy containing 38-46wt.% Ni is welded on the inside surface of the outer layer 1 molded by steel material is held freely movably by a guide 11, the complicated pipe 3 is passed through a high frequency coil 12 arranged near the termination of the guide 11, and bent by a fixed radius as the complicated pipe 3 is heated locally and delivered to the side of the high frequency coil 12. In this case, the temperature on the inside surface of the back side is heated to 750-900°C while the delivery speed does not exceed 0.5mm/sec.