A clamping device is attached to a front part of a body of a boring machine for clamping the boring machine and a workpiece together. The clamping device comprises a fixed piston fixed on the front part of the body, a movable piston equipped over the front part of the body movably in the axial direction of a spindle of the boring machine, a first pressure chamber, a second pressure chamber, a pressurized fluid-supply device to supply pressurized air to the first pressure chamber and the second pressure chamber so that the movable cylinder is moved in the axial direction, a connecting mechanism an end portion of which is connected to the movable cylinder, and a clamping unit which is connected to the other end portion of the connecting mechanism, and is fixed to a certain position of the workpiece.