To realize the firm welding with relatively small beads by providing at least a metallic pipe frame on one side which is opposite to other metallic frame.

An axial part of a pipe frame 12 opposite to other metallic frame 10 is recessed in the radial direction along the axial direction from the part about one half of the circumferential surface to form the axial part of approximately crescent section, and the pipe frame 12 is opposite to the other metallic frame 10 on the inner surface side of the recessed axial part, and beads 16a, 16b are provided along two valleys which are tangential lines of the frames.