PURPOSE: To mate easily the surfaces of the welded joints of two cylindrical bodies in the stage of butt-welding both cylindrical bodies by using a surface- mating ring provided with a seat for receiving the surface-mating ring attached to the one cylindrical body and a surface ring provided with a butt-guide attached to the other cylindrical body.

CONSTITUTION: A mounting ring 11 is attachably and detachably mounted by means of bolts 12 to the welding end part of a cylindrical body 1A in the stage of butt-welding the cylindrical bodies 1A and 1B. A surface-mating ring 15 provided with plural guides 14 for butting the cylindrical bodies is attached to the upper flange of a seat 13 for receiving the surface-mating ring attached to the outside circumferential surface of said ring 11 and a hole 19 for inserting a spacer 18 for holding a groove gap is pierced to the web part thereof. The body 1B is stacked on the body 1A with said guides 14 as a guide and the dislocating surfaces of both bodies 1A, 1B are mated and thereafter the spacer 18 is removed while the rear side of the groove is welded so that the entire circumference is tack-welded.