PURPOSE: To prevent the cracking of the outer shell and the generation of the defective joining by heating an outer tube immediately before achieving the incline rolling so as to be higher in temperature compared with an inner tube by an auxiliary heating means provided on an incline rolling equipment.

CONSTITUTION: When an outer tube made of Ni-based alloy and an inner tube made of steel which are uniformly heated to the forming temperature range are carried in an incline rolling equipment 13, the outer tube is rapidly cooled by the heat release in the atmosphere, and the difference in the distortion resistance between the outer tube and the inner tube is increased. In the auxiliary heating process, the outer tube immediately before the incline rolling is heated again so as to be higher in temperature compared with the inner tube by an auxiliary heating means 18 such as a high frequency induction heating coil 30 provided on the incline rolling equipment 13. As a result, the difference in the distortion resistance between the outer tube and the inner tube is reduced, the material flowability of the joining interface in the rolling process becomes excellent, and failures such as the cracking of the outer shell and the whole or partly peeling of the joined part are suppressed.