PURPOSE: To attain high alignment precision without adjusting by giving a number of degrees of freedom in the radial direction to a collet body when the collet body reaches a small-diameter part of a guide member on the way of movement in case that the collet body is fitted to a driving member.

CONSTITUTION: When a slide member 33 is slid to a halfway alignment state, a guide slope 23 of a collet 23 strikes against an eccentric disc center hole 27 to start aligning a disc 24, and the slide member 33 is slid on a guide member guide part 32-1 till reaching this state. When a collet body pressing member 21 falls, the slide member 33 reaches a guide member small-diameter part 32-2. Since the slide member 33 gets a number of degrees of freedom in the radial direction with respect to the guide member small-diameter part 32-2 then, the center deviation between a guide member 32 and a disc driving member 25 is absorbed. Next, a collet expanding member 28 is slid on a slide slope 9 of the collet 22 to absorb the variance of the disc center hole 27, and a disc 24 is aligned and clamped thereafter.