To provide a holding device of an optical disk constituted so as to increase the storage capacity for the small diameter disk by appropriately adjusting the size of the clamp area in accordance with the diameter of the optical disk.

The device includes a projecting part 21b whereon a center hole of the optical disk is fitted, a shoulder part 21a whereon the peripheral edge of the center hole of the optical disk is placed around this projecting part 21b, a turntable 21 rotationally driven by a rotation driving means, an adapter ring 22 attachably/detachably mounted against the turntable 21 to form the placing surface of the large diameter optical disk at the time of mounting, and optical disk holding parts. 23, D2a, D3a for pressurizing the optical disk against the optical disk placing surface, then in the case of small diameter optical disk, the optical disk is supported with the shoulder part of the turntable 21 in the state when the adapter ring 22 is detached, and in the case of large diameter optical disk, the optical disk is supported with the optical disk placing surface of the adapter ring 22 in the state when the adapter ring 22 is mounted.