PURPOSE: To maintain a specified spacing between inside and outside tubes and to bend the tubes to a prescribed shape by confining granular materials, such as sand, into a cylindrical insertion part of a forming tool and bending the tubes in the state of averting the direct contact of these materials with the tubes.

CONSTITUTION: The inside and outside tubes 2, 3 to be worked are fitted to each other and one-side ends thereof are crushed. The inside and outside tubes 2, 3 are integrated by end face welding, etc. The granular materials 8 existing in the cylindrical insertion part 6 of the forming tool are once moved into a recovering bag part 7 and the emptied cylindrical insertion part 6 is inserted into the clearance in the fitting part between the inside tube 2 and the outside tube 3. In succession, the granular materials 8 in the recovering bag part 7 are moved into the cylindrical insertion part 6 and are fixed by using a fixing means 11, by which the granular materials 8 are prevented from returning into the recovering bag part 7. The inside tube 2 and the outside tube 3 are simultaneously bent by using a bender, etc. As a result, the efficiency of bending of the double tubes is enhanced and the double tubes are formed without crushing the spacing between the inside tube and the outside tube.