PURPOSE: To improve the strength at a joining boundary and to prevent the peeling between plating layers by putting a tube onto a bar subjected to grooving and subjecting the projecting parts of the bar and the contact surface of a tube to diffusion joining by an HIP method.

CONSTITUTION: The projecting parts of the parts 1 subjected to the grooving and the inside surface of the tube 2 are subjected to the diffusion joining by the HIP method and, therefore, the joint strength at the boundary thereof is improved and the peeling of the plating layers generated in the case of production by the conventional method is prevented.