PURPOSE: To ensure the accurate positioning of the clamp member with a simple mechanism and to realize the stable revolution of a disk by adding a cam part to the contact part between a clamp member and a mobile member to reduce the fitting range between both members.

CONSTITUTION: A projection part 19b of a clamp member 19 is partly defined as a conical cam part 19c increasing its width toward its upper side. A fitting part 12a of a mobile member 12 can have a contact with the upper part of the part 19c. Therefore a sufficient space is secured between the part 12a and the part 19b at revolution of the disk D. When the disk D is loaded, the member 12 touches the part 19c when touching a support member 13. Thus the position of the member 19 is regulated. In such a constitution, the member 19 can be accurately positioned and the revolution of the disk D is stabilized in a simple constitution.