A grip member is inserted into an insertion hole of a cylindrical clamp body and is capable of extending outwards to grip the inner circumferential surface of an aperture of an article. A clamp rod is engaged with the grip member; a driving mechanism capable of driving the clamp rod forwards and backwards; a seating surface formed at the top end of the cylindrical clamp body; a case member supporting the cylindrical clamp body movably forwards and backwards; a driving mechanism capable of raising an lowering the cylindrical clamp body; and a lock mechanism having a cylindrical member into which the cylindrical clamp body is inserted so as to slide freely and capable of locking the outer circumferential surface of the cylindrical clamp body by elastically deforming the cylindrical member so as to reduce its diameter.