An adapter A1 made of a synthetic resin includes an annular frame 1 having an outside diameter of 12 cm, a disk receiving section 2 provided continuously on a lower portion of an inner edge of the frame 1, and a pair of disk pressing sections 3 and 3 provided continuously on an upper portion of the inner edge of the frame 1. The disk receiving section 2 is provided with a circular opening 4 for causing an information recording area of a disk to face an optical reader of a disk player. A disk press-contact projection 7 formed so that a projection height is gradually increased in a disk rotation direction Y is provided on a lower surface of each disk pressing section 3. This adapter A1 can securely install a non-circular optical disk even if the optical disk has a slightly different size, and an installation operation of the non-circular optical disk can be easily performed.