PURPOSE: To perform easily and thoroughly diffusion joining of a steel and a metallic plate by superposing the metallic plate on the surface of the steel, subjecting the circumferential edge of the mated surfaces to seal welding, maintaining the spacing between the mated surfaces in a non-oxidizing atmosphere and heating and holding the assembly while exerting pressing force thereto from the outside.

CONSTITUTION: A metallic plate 2 to be clad and joined is superposed on the surface of a steel 1 and the entire circumference 4 at the circumferential edge of mated surfaces 3 is subjected to seam welding. The spacing between the surfaces 3 is then sucked to a vacuum through a pipe 6 provided to the plate 2 or an inert gas is sealed therein to maintain said spaciang in a non-oxidizing atmosphere and thereafter the assembly is put in a hermetic vessel 5 for pressurization. The inert gas is fed through a feed pipe 7 into the vessel 5 to maintain the pressure in the vessel under about 10W30kgf/mm2 and while pressing force is exerted to the surfaces 3 from the outside, the assembly is heated and held at a required temp. by a heating coil 9. The steel 1 and the plate 2 are throughly diffusion-joined at the surfaces 3 by such treatment and the cladding of the plate 2 free from the non-joined part is thus obtd.