PURPOSE: To improve productivity and to reduce manufacturing costs in manufacture of a tube having a radiator fin through welding by utilizing welding residual heat to plow the outer layer of the tube outwards with a cutter and to form the layer into the fin.

CONSTITUTION: A long metallic flat band 1 is formed into a cylindrical shape with roller 2W5, and is heated by a high frequency induction coil 6 and welded on the seam of the band formed with a press fitting roller 7 to produce a metallic weld tube 8. A cutter 9 integral with a dice 10 plows the outer layer of the tube 8 outwards while the tube 8 is supported with a friction roller 14, and is heated up to a high temperature through residual heat of welding and the plowed layer is passed through the dice 10 for forming the layer into a radiator fin through bending and folding. In this case, a metallic guide B is inserted inside of the tube and water discharged from the front end of the guide B cools the inner surface of the tube to prevent deformation through pressure caused by the cutting action of the cutter to provide a high operating efficiency in a manufacture of a tube having a radiator fin and reduction of manufacturing costs.