The flexible ring fitting device includes a flexible ring disposition mechanism 10 for disposing a flexible ring R relative to an annular groove G of a workpiece W and flexible ring assembly mechanisms 421 and 422 including an arm portion 424 rotating while pushing the flexible ring R along the outer periphery of the workpiece W. The flexible ring assembly mechanisms 421 and 422 are movable with respect to the workpiece W and include an elastic body 51 urging the arm portion 424 of the flexible ring assembly mechanisms 421 and 422 so as to press the arm portion 424 with respect to the workpiece W.