An O-ring mounting device 1 has an O-ring support unit 15, 16 capable of supporting an O-ring from its inside at at least three support points, an O-ring pressing-in unit 14, 27 for pressing-in the O-ring from its outside between two support points of the at least three support points; and an pressed-in state releasing unit 23 for releasing a pressed-in state of the O-ring made by the O-ring pressing-in unit 14, 27. An O-ring mounting device which is applicable in a wide range without being restricted by a dimension of the O-ring or the like can be provided.