To simply attach tongues and/or projecting parts to a thin sheet and to form a rectangular tube with the member attached such tongues and/or projecting parts.

The rectangular tube 10 for an exhaust has heat transmitting material is assembled with two U shape thin sheet shells 11, 12, and tongues 13, 14 are projected from the bottom of the thin sheet shells 11, 12 into the rectangular tube 10. Tongues 13, 14 are formed by solid forming, especially extruding from the bottom of the thin sheet shells 11, 12. A pressure is flatly applied so that the thin sheet fragments formed as the thin sheet shells 11, 12 exceed the yield point of the thin sheet material between a female die and a pressing member and moreover a part of the thin sheet material flows into the slit of the female die (or the pressing member) determining the shape of tongues 13, 14.