To enable welding with a laser beam and plasma so as to improve the workability in assembling by increasing adhesive strength of an outer shell to the outer peripheral surface of an end plate without generating deformation on an intermediate assembly.

A cylindrical outer shell 1 is formed by inserting an intermediate assembly 2 in which inner pipes 23, 24 are fixed to inner plates 21, 22 and also the end plates 25, 26 are fixed to both end sides into the outer shell 1 which is preliminarily formed into a cylindrical shape and welding edge parts of a metallic plate 1a which comprises the outer shell 1 along the axial direction in a state wherein the outer shell 1 is closely adhered to the outer peripheral surfaces of the inner plates 21, 22 and the end plates 25, 26 with a chain 4 for fastening the outer shell 1 in the axial direction by enclosing the outer peripheral surface of it and also both open edge parts at both ends of the outer shell 1 and the outer peripheral edge parts of the end plates 25, 26 are welded over the entire periphery with the laser beam.