To obtain a high precision clamp device which can avoid the possibility of complete overlapping of errors by positional allowance of the clamp device and of errors by incompleteness of the surface to be held by the clamp device.

A clamp device has two jaws, and a work piece to be machined is fixed on a table of a machine tool between two jaws. Each jaw is supported by carriages 1 and 2, and the carriages are linked so that they can move in a way to be orthogonal to a passage of the carriage and to be mirror image symmetrical on the static plane to the device. Hence the addition of a positional tolerance of the clamp device to the machine tool and the tolerance of shapes of the work piece, can be avoided.