PURPOSE: To highly precisely form a seat where the seat for attaching parts between bent parts of a bent double tube.

CONSTITUTION: Seats are formed respectively on internal and external tubes 11, 14 in the straight tube state. Next, the internal tube 11 is inserted into the external tube 14 and both seat parts 13, 15 are mutually joined, and a filler 17 which is possible to be taken out is filled inside a clearance 16 between the internal and external tubes 11, 14, then the internal and external tubes 11, 14 are bent, then, parts attaching holes 19, 20 are formed on the seat parts 13, 15, and the filler 17 is taken out. The seat parts can be precisely formed by using the seat parts 13, 15 as an inner die in the straight state of the inner and outer tube 13, 15.