PURPOSE: To protect the inner circumferential surface from breakage by providing a projection at a part of a spindle and changing frequently the diameter of the medium holding position of a clamp with a part of the clamp defined as a fulcrum when the clamp is shifted toward the spindle.

CONSTITUTION: A clamp mobile shaft 7 is fixed to a cone 6 at its end of one side and has a hollow part 9 at the lower part of a clamp 8. A projection 10 of a spindle 1 gives deformation to the clamp 8 at an area corresponding to the cone 6 in response to the part 9 and at the same time receives the pressing force of the cone 6 when the clamp 8 corresponds to the spindle 1. In such a way, the clamp 8 is pressed down by the cone 6 through the shaft 7 and can be deformed with a part 8' of the clamp 8 corresponding to the cone 6 defined as a fulcrum before the clamp 8 is set oppositely to the spindle 1. Then the part 8' is extended to the outside by the cone 6. Thus the part 8' has close contact with the inner wall of the spindle 1, and the inner diameter of a medium 2 is set oppositely to tht of the spindle 1 from the lower part. Then the medium 2 is clamped between the clamp 8 and the spindle 1. This device protects the inner circumferential surface of the medium 2 from damage.