A pair of locking members (22) (22) protruding upward relative to a housing (1) and configured to be inserted into a hole of a workpiece are arranged to be capable of being respectively pressed onto right end left portions of an inner peripheral surface of the hole. Wedge surfaces (33) (33) of a clamp rod (32) are respectively engaged with the locking members (22) (22) from above. The clamp rod (32) is configured to be driven by a driving means (D) downward for clamping and upward for unclamping. The clamp rod (32) includes recesses (50) (50) formed, in an outer peripheral surface of the clamp rod (32), at a portion of the clamp rod (32) which is located below the wedge surfaces (33) (33). The recesses (50) (50) are configured to respectively accommodate the locking members (22) (22) in such a manner that inner lower portions of the locking members (22) (22) in an unclamping position are allowed contact each other.