To provide a roll-forming round die without loosing between an annular die part at the outer peripheral side and a core part at the inner peripheral side at a low cost.

The roll-forming round die 10 is fitted to the outer periphery of a base part 20 in the state of expanding the metallic annular die part 24 by heating. Since the annular die part 24 having the inner peripheral surface 22 of the diameter smaller than the outer peripheral surface 18, is firmly fitted to the outer peripheral surface 18 of the base part 20 by shrinkage-fitting the annular die part 24 shrunk with the cooling in this state, the annular die part 24 at the outer peripheral side and the core part 20 at the inner peripheral side, are firmly connected without the screw formation for screw connection and loosing preventing mechanism. In the case the wear and the defect are caused, since only the annular die part 24 can be exchanged, the roll-forming round die 10 is obtained in a low cost.