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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

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.

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The invention claimed is: 1. In an outer tube of an exhaust system part, with a sheet metal being rolled double to form a cylinder having an inner plate and an outer plate, and form an overlapped portion with circumferential end portions of said inner plate and outer plate crossed in the circumfere

The invention claimed is: 1. In an outer tube of an exhaust system part, with a sheet metal being rolled double to form a cylinder having an inner plate and an outer plate, and form an overlapped portion with circumferential end portions of said inner plate and outer plate crossed in the circumferential direction, and being joined at the overlapped portion, the outer tube of the exhaust system part characterized in that a plurality of circumferential communication passages are formed between said inner plate and said outer plate, with tip end portions of the respective circumferential communication passages facing with each other to be apart from each other by a predetermined distance from a peripheral edge of one of said inner plate and said outer plate, and are arranged in parallel with each other along a longitudinal direction of said outer plate, and that at least one portion of said overlapped portion positioned between neighboring circumferential communication passages out of said plurality of circumferential communication passages is joined. 2. An outer tube of an exhaust system part as set forth in claim 1, wherein radial cross sections of said inner plate and outer plate are formed in a non-circular shape, respectively, and wherein said circumferential communication passages are formed at positions of said inner plate and outer plate having a minimal radius of curvature thereof, respectively. 3. An outer tube of an exhaust system part as set forth in claim 1, wherein said circumferential communication passages comprise a plurality of line grooves formed by bulging said outer plate out. 4. An outer tube of an exhaust system part as set forth in claim 1, wherein the one portion of said overlapped portion positioned between the neighboring circumferential communication passages out of said plurality of circumferential communication passages is welded. 5. In an outer tube of an exhaust system part, with a sheet metal being rolled double to form a cylinder having an inner plate and an outer plate, and with circumferential end portions of said inner plate and outer plate being joined by a lock-seaming process, the outer tube of the exhaust system part characterized in that a plurality of circumferential communication passages are formed between said inner plate and said outer plate, with tip end portions of the respective circumferential communication passages facing with each other to be apart from each other by a predetermined distance from a peripheral edge of one of said inner plate and said outer plate, and are arranged in parallel with each other along a longitudinal direction of said outer plate. 6. An outer tube of an exhaust system part as set forth in claim 5, wherein radial cross sections of said inner plate and outer plate are formed in a non-circular shape, respectively, and wherein said circumferential communication passages are formed at positions of said inner plate and outer plate having a minimal radius of curvature thereof, respectively. 7. An outer tube of an exhaust system part as set forth in claim 5, wherein said circumferential communication passages comprise a plurality of line grooves formed by bulging said outer plate out.