To provide a ring body forming device which is not affected by the backlash or temperature difference but form a work of high circularity, high accuracy and high quality by providing a movable member to move a forming roller in the pressing direction, and performing screw-feed control of the movable member by a screw driven by an NC servo motor.

This ring body forming device rotatably and pivotably supports three rotational elements 30, 27 and 24, i.e., a forming roller 30 having a ring body forming unit on a circumferential surface thereof, a mandrel 27 which faces the forming roller 30 in an attachable/detachable manner having a ring forming unit and a regulation unit formed on a circumferential surface of an intermediate part and on both circumferential surfaces, respectively, and a support roller 34 which is provided facing a back side of the mandrel 27, and has a slidable contact part facing and in contact with the regulation unit of the mandrel. A movable member 6 moves the forming roller in the pressing direction is provided, and a control means performs the screw-feed control of the movable member 6 by a screw 7 driven by an NC servo motor 10.