There is a workpiece clamp device 10 for horizontally holding a thin section long workpiece 1 having a constant section shape or having the same section shape in spaced positions in a longitudinal direction in two positions of the same section shape. The workpiece clamp device 10 is constituted of a driving clamp device 10A and a driven clamp device 10B comprising the same holding device 12. The driving clamp device 10A rotates the workpiece 1 centering on a horizontal axis O extending in a longitudinal direction of the workpiece, and the driven clamp device 10B follows movement of the workpiece and idles centering on the horizontal axis. An arbitrary portion to be machined of the workpiece can be directed in a direction in which the portion is easily machined by the rotation (e.g., upward direction).