To prevent breakage of a workpiece and a tool when the power supply to a clamp device is stopped by suppressing free rotation of a main spindle in a non-load state.

A slide disk 43 formed displaceable toward the side of the base section 11 of a frame 2 by deflection is provided on the side of the main spindle 4a, and a friction surface 44 capable of contacting a pressing portion 43b of the slide disk 43 is provided on the side of the base section 11. By always applying a pressing force imparting a rotational resistance allowing the rotation of the main spindle 4a to the main spindle 4a by always pressing the pressing portion 43b of the sliding disk 43 to the friction surface 44, the rotational resistance can be applied to the main spindle 4a, even if the position of the center of gravity W of a member to be rotated is deviated from a rotational axis line 205 of the main spindle 4a.