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

CONSTITUTION: The disk adapter A has an outer peripheral surface 1 nearly in the 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. Then the movable engagement part 4 which is elastic is formed facing the opening hole so that it can reciprocate in the radial direction of the opening hole, and a lock part which locks the small-diameter disk is formed at the movable engagement part. A couple of fixed engagement parts 6 which engage the small-diameter disk are formed on opposite sides of the opening hole which face the movable engagement part. Then the distance between the movable engagement part and fixed engagement parts is set longer than that between the fixed engagement parts. The disk adapter is symmetrical with respect to the straight line L connecting the intermediate position of the couple of fixed engagement part and the center of the opening hole.