PURPOSE:To enable the easy manufacture of a smooth circumferential joint by directing the frame, which is mounted with one pressure roll in precedence to a welding torch and each one on the right and left and the torch, to the weld line direction by a rotation-driving means. CONSTITUTION:Three rolls 5, 6, 6 are provided on the bottom surface of the frame 19 suspended to a pipe making device frame 18 by way of a hydraulic cylinder 10, rotary frame 11 and support rod 29, and a welding torch 7 is fixed on the centerline of the triangular spacing formed by said rolls 5, 6, 6. When a steel strip 2' being coiled in the state having camber comes, oil pressure is exerted to the top of the rod of the cylinder 10 to lower the frame 19 and let the rolls 5, 6, 6 contact the steel plate 2'. Further, oil pressure is applied to hydraulic cylinders 29, 23, 23 to press the rolls 5, 6, 6 onto the steel plate 2' to eliminate camber and allow the steel plate make sufficiently close contact with the underlayer, after which welding is started, whereby the steel plate 2' being subsequently coiled in spiral form is continuously automatically welded.