PURPOSE: To maintain reliability of insertion welding and simplify the work by inserting a tube provided with a projection on the surface of the tube into a tube stand before welding.

CONSTITUTION: A projection 6 is provided on the surface of a tube 2 at l-g (l; depth of insertion of tube stand, g; intial space between the tube and tube stand) from the end of the tube. The projection 6 is made by, for instance, forming after making build up welding on the surface of the tube. Next, by inserting the tube 2 provided with the projection 6 into the tube stand 1 by l-g, the initial space (g) can be secured accurately. Consequently, initial space is secured perfectly and groove fitting work is made easy. At the same time, tack welding and first layer welding are made easy as the projection on the surface of the tube is made a filler metal.