To improve joining quality so that friction welding can be performed by properly holding heat balance in an abutting part between a tube and a cover.

In the abutting part between the tube 1 and the cover 2, there is formed an annular projection 10 having nearly the same cross section size as the tube 1. Friction welding is performed by abutting this annular projection 10 and the tube 1, wherein a flash 13 then produced is released to a release groove 12 preliminarily formed around the boss 3 of the cover 2. Since the annular projection 10 and the tube 1 having a roughly equal thickness are friction welded, heat balance is improved in the abutting part between the cover 2 and the tube 1. As a result, a nearly equal amount of flashes 13, 14 is produced from the cover and the tube, enabling improved joining quality to be obtained.

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