Abstract: An apparatus for continuously producing electric-welded steel pipes by continuously supplying coiled strips as connected to one another by welding, progressively forming the strip into a tubular shape and high-frequency welding the opposed edges of the strip to form a seam. To produce required small quantities of steel pipes having varying diameters and wall thicknesses, a group of roll assemblies is replaceable by another group within a short period of time. Groups of forming, welding or sizing rolls are mountable on a polygonal bed which is lockable in position after rotation. For replacement, the bed is rotated by a hydraulic motor to bring the desired group of rolls to the proper position quickly.