A clamp device includes a grip member, a clamp rod, and a hydraulic cylinder, and, by the grip member being inserted into an hole in a workpiece, it can grip the inner circumferential surface of the hole. The clamp device is provided with: a main body member; seating surfaces formed upon the main body member, upon which the workpiece is seated; a seating sensor that includes a pressurized air ejection hole opening to the seating surfaces; and a workpiece mounting surface that can receive and stop a workpiece at a position that is more advanced than the seating surfaces when the grip member is at an advanced position (its unclamped state), and that shifts together with the grip member during clamp operation when the grip member and the clamp rod shift towards retraction.