A forming device having a rotating frame (1) and a forming blade (2) movable in the axial and radial directions of the rotating frame. One side-edge portion of a tubular blank (B) is fixed to a predetermined position of the rotating frame (1), and the other side-edge portion is movably set on the rotating frame. While the rotating frame (1) is rotated, the forming blade (2) is pressed to the inner diameter side or the outer diameter side of a peripheral wall of the tubular blank (B). The forming blade is reciprocated from the fixed side-edge portion radially outward or inward and moved toward the movable side-edge portion. Thus, a tubular formed body in which at least a ridge portion or a recess portion continuing in circumferential direction is shaped in the peripheral wall is formed.