To provide a clamp device adapted to hold a non-elliptical cylindrical workpiece material without distortion, and directly hold a distorted non-elliptical cylindrical workpiece material, and finally machine them into a distortion-free elliptical cylindrical shape or optional non-elliptical cylindrical shape.

The clamp device 100 includes: an installation section 2 to install a bottom surface 1A of a cylindrical workpiece and a circular workpiece 1 or the like by dividing them into multiple sections; a lift drive mechanism 20 to individually raise and lower installation surfaces 2A to 2H; a clamp mechanism 10 which is disposed at an outer peripheral surface position 2Y of each of these installation surfaces of the installation section, and expands and reduces in an outside diameter direction; and a pressure control mechanism 30 to control and apply an abutting pressure uniformly onto the bottom surface 1A and an outer peripheral surface 1B of the cylindrical workpiece with respect to the lift drive mechanism and the clamp mechanism.