PURPOSE: To manufacture a metallic welded tube having a good weld zone by applying a lubricant whose specific gravity is made constant by replenishing a solvent on the underside of a metallic strip before forming, forming with a die and welding it.

CONSTITUTION: The lubricant 2 is applied on the metallic strip 1 which is delivered from an uncoiler with an applying roller 3. After forming into a tubular shape by passing the metallic strip 1 through the die 4 having a tapered through hole 5 of a circular section, the welded metallic tube is manufactured by welding the butted part with a welding torch 8. At that time, the lubricant 2 whose specific gravity is made constant by continuously or intermittently replenishing the solvent is applied on the whole of the underside of the metallic strip 1 before forming with the die 4. The specific gravity of the lubricant 2 is measured with a standard specific gravity meter 11 in a specific gravity measuring part 10 and the solvent is replenished from a solvent supplying port 12. In this way, the occurrence of weld defects such as a blow hole in the interior of a weld bead is prevented.