PURPOSE: To simplify the manufacturing stage and to reduce the manufacturing cost by welding and joining partially in advance the periphery of a joint and heating it up to the rolling temperature by a heating furnace which is replaced with inert gas and whose residual oxygen is kept under the specific quantity and afterward, performing the hot rolling.

CONSTITUTION: Base metal 1 is combined with clad material 2 and the base metal 1 and the clad material 2 are joined and integrated by the welding. The joint is heated up to the hot rolling temperature by the heating furnace whose inside is replaced with the inert gas and residual oxygen concentration is made to ≤150ppm and afterward, the hot rolling is performed. By this method, the size of a product is enlarged and the manufacturing stage is simplified and the manufacturing cost is reduced.