To automatically and internally fitting a ring into a work by providing enlarging elements which enlarge an elastic ring held into an approximately heart shape by a pin from the inside and fit it into a prescribed position of the work.

A butt plate 29 advances to dent an elastic ring 31. Then, the ring 31 is regulated by a roller 16, a presser plate 23a, and the butt plate 29 so that its diameter is contracted into an approximately heart shape. With the advancement of the butt plate 29, a pin 8 is entered into its groove 29a to hold the approximately heart shape of the ring 31. A mount board 10 is raised to set a work 32 on the mount board 10. Then, the pin 8 is lowered, two enlarging elements 23, 24 are moved in the outer circumferential direction respectively, and the ring 31 is enlarged by the presser plates 23b, 24b and fitted in a fitting position 32a of the work 32. In this case, the enlarging elements 23, 24 enlarge a part of the ring 31 butted by the butt plate 29.