Abstract: To promote the forming performance of band material into pipes and to decrease the number of forming stages and changes of the forming rolls in the pipe mill, considering that it is superior when forming the band material into pipes of required diameters if the sectional shape of the band material is provided with a curved surface which changes its curvature continuously into the required diameter, we provide a curved roll surface of numerous curvatures changing continuously. In a continuous roll forming in accordance with the present invention, a forming roll having an approximate involute curve based upon a polygon comprising a plurality of sides is used. By using the conventional rolls in various combinations therewith in each stage of forming as a horizontal roll, side roll and upper intermediate roll or a cage roll, the number of stages of the forming roll can be reduced.