PURPOSE: To provide a manufacturing method and device for manufacturing a curved bent tube which can seam the flange integrated to the matching surface of a work.

CONSTITUTION: Two segmented works are held together and seamed by falling both flanges of the inner and outer circumferential walls to the side continuously from the end parts. A frame plate 10 which is erected parallel to the segmented surface of the work is provided, and a long guide hole 15 of the prescribed width along the work segmented surface is pierced, a driven arm 20 is arranged on the work side of the frame plate, and a pin 21 to be slidably fitted in the oblong guide hole is connected to the driven arm. A driving arm 30 which is provided with a long hole 33 where the pin is fitted to the counter-work side of the frame plate 10 and is turned by a driving means 40 is pivotably supported, and a guide part 50 to guide the movement of the driven arm 20 is provided on the frame plate 10, and a roller to press the flange of the work is pivotably supported by the driven arm.