Abstract: A disk playback apparatus with a centering mechanism for positioning a small or a large disk which comprises a support frame, a clamper arm swingably mounted on a rear portion of the support frame, and a centering mechanism for guiding large and a small diameter disks from a front side of the playback apparatus to a position between the support frame and the clamper arm when the disk is inserted into the playback apparatus. The centering mechanism has a pair of centering arms for contacting with a circumferential edge of the disk for positioning the disk in place, a centering lever provided in the clamper arm and connected to the pair of centering arms for simultaneously opening/closing the pair of centering arms, and lock arms each rotatably mounted on the associated centering arm for locking the centering arm independently, the lock arms allowing the pair of centering arms to be swingable when both the lock arms are released.