PROBLEM TO BE SOLVED: To provide a diameter reduction device capable of excellently changing an outer diameter of a diameter reduction object without biting in a part of the diameter reduction object.SOLUTION: An opening part 4b is constituted by combining a tip part 4a of a plurality of top bodies 4. The plurality of top bodies 4 rotate around respective fulcrum support pins 8 by rotating a ring 4a, and are constituted so as to change an inner diameter of the opening part 4b by moving in the radial direction by coordinating the tip part of the plurality of top bodies 4. Assuming a diameter of a circle of connecting the center of a plurality of fulcrum support pins 8 as PCD and a distance up to the tip part 4a of the top bodies 4 held by the fulcrum support pins 8 from the center of the fulcrum support pins 8 as L0, L0/PCD is larger than 0.5.SELECTED DRAWING: Figure 8
