PURPOSE: To obtain the disk adapter which can securely hold a small-diameter disk only by pressing the small-diameter disk against the movable engagement part of the disk adapter.

CONSTITUTION: The disk adapter A has an outer peripheral surface 1 nearly in the same circular shape with a large-diameter disk and also has an opening hole 2 wherein the small-diameter disk CD is to be fitted, formed at its center part. The opening hole consists of a 1st semicircular opening hole part 21 and a 2nd opening hole part 22 having the same diameter r1 with the small-diameter disk and a connecting opening hole part 23 which connects the 1st and 2nd opening hole parts. Therefore, the small-diameter disk is movably in the opening hole. At the center part of the outer periphery of the 2nd opening hole part, the movable engagement part 4 which is elastic is arranged movably by reciprocation in the direction to the center O of the outer peripheral surface so that the distance to the center of the outer peripheral surface is nearly equal to the radius of the small-diameter disk. A 2nd engagement part 6 is formed at the position facing the movable engagement part.