A sheet metal is rolled double to form a cylinder having an inner plate 2 and an outer plate 3, and form an overlapped portion 4, with their circumferential end portions 2a and 3a crossed in the circumferential direction. Between the inner plate 2 and the outer plate 3, a plurality of line grooves 5 are formed to extend outward of an outer tube 1, with their tip end portions facing with each other to be apart from each other by a predetermined distance from a peripheral edge of one of the inner plate and outer plate, and these line grooves 5 are arranged in parallel with each other along a longitudinal direction of the outer plate 3. And, at least one portion of the overlapped portion positioned between the neighboring line grooves is joined by welding W.