A clamp device includes an annular grip member capable of being inserted into an hole in a workpiece and of gripping the inner circumferential surface of the hole, a clamp rod having a tapered shaft portion fitted inside the grip member and engaged therewith, a hydraulic cylinder capable of axially advancing and retracting the grip member and the clamp rod, and a clamp main body member. The clamp device is provided with seating surfaces formed upon the clamp main body member, on which the workpiece is seated, and a poor clamping detection mechanism that detects poor clamping, in the state in which the grip member is driven in its clamping direction by the hydraulic cylinder via the clamp rod, and the workpiece is in its state of being seated upon the seating surfaces.