PURPOSE: To eliminate a seam guide roll and to attain shortening of a production line by controlling the position of both side edges of a metallic belt processed for bending, making it pierce through a die and welding the abutting part of both side edges immediately with a welding torch.

CONSTITUTION: The metallic belt 1 is applied with a lubricant on the surface that comes into contact with the die by a lubricant applicator 2 and subjected to a bending process from both side edges 1a to the part of a prescribed width by an edge bend roll 3. Next, by a guide mechanism 11, both side edges 1a are controlled for positions. Then, immediately, by the die 4 having a taper 4a in which the diameter becomes smaller from the entrance side to the exit, the belt is formed into a tube with both side edges abutted. Then, the abutting part 1b is butt-welded immediately by a squeezing roll 6 and welding torch 6, and a metallic welded tube 8 is obtained. Thus, the yield of product is improved.