PURPOSE: To improve fatigue strength of a joint by forming a recessed part on a boundary part between titanium of clad steel and carbon steel and filling inert gas thereinto and then, subjecting the butt joint of titanium to penetration welding.

CONSTITUTION: Single-V grooves 5 are formed respectively on the surface of the titanium 2 side of the butt joint and the rear of the carbon steel side 3 and the recessed part 11 is provided to the boundary part. Carbon steel 13 is first welded and then, the butt joint 12 of titanium is subjected to penetration welding while inert gas being supplied to the recessed part 11 from a telltale hole 9. By this method, since titaniums and carbon steels can be subjected to butt welding mutually separately, the fatigue strength of the joint with respect to fatigue due to thermal expansion, etc., is remarkably improved.