PURPOSE: To prevent the generation of wrinkles and to improve the forming accuracy in bending the peripheral end part of a circular ring, by clamp-pressing the metallic circular ring between a circular ring driving roll equipped with a working roll and a circular ring driving roll equipped with a cushion roll.

CONSTITUTION: A work 13 is put between an outside work-driving roll 3 and an inside work-driving roll 4 rotating at the same speed. Next, the work 13 is pressed between a top and bottom surface processing work roll 1 of the roll 4 and a top and bottom surface processing cushion roll 2 of the roll 3, and the roll 1 is pushed forward to the direction of the roll 2 against the resistance of a definite pressure of the roll 2. Then, a peripheral edge processing part of the roll 1 is pushed forward to the peripheral edge-part of the roll 3 with retreat of the roll 2, to press the clamped top and bottom end parts of the periphery of the work 13 to the outside direction, thereby performing an automatic hemming at a stroke. Further, in a case of hemming the peripheral top and bottom parts of the work 13 to the inside direction, the driving rolls 3 and 4 and working rolls 1 and 2 are rearranged properly, or the design is changed at one's option.