A chuck comprising: a plurality of jaws in front of a chuck body, the jaws sliding radially to support a workpiece so that a center of the workpiece agrees with a rotation axis (Jr), wherein each of the jaws includes: a master jaw assembled to the chuck body and having, in front of the chuck body, a contacted surface with serrations extending circumferentially; a connecting member being inserted into a support groove that is formed in the contacted surface to extend radially and being incapable of coming out along the rotation axis (Jr); and a top jaw having a contact surface with serrations, fastened to the connecting member along the rotation axis (Jr) to come into close contact with the contacted surface, and being detachably attached to the master jaw to support the workpiece, the connecting member includes: a stopper extending along the support groove; and a key extending, with a width smaller than that of the stopper, in accordance with the stopper; the support groove includes: a first keyway into which the key is fitted with a tip of the key protruding from the contacted surface; and a stopper retainer configured to hold the stopper, the top jaw includes, on the contact surface, a second keyway into which the tip of the key is fitted, characterized in that a gap exists between the key and each of the first keyway and the second keyway, and a close contact between the contact surface and the contacted surface inclines or twists the key with respect to both the first keyway and the second keyway. each of the jaws includes: a master jaw assembled to the chuck body and having, in front of the chuck body, a contacted surface with serrations extending circumferentially; a connecting member being inserted into a support groove that is formed in the contacted surface to extend radially and being incapable of coming out along the rotation axis (Jr); and a top jaw having a contact surface with serrations, fastened to the connecting member along the rotation axis (Jr) to come into close contact with the contacted surface, and being detachably attached to the master jaw to support the workpiece, a master jaw assembled to the chuck body and having, in front of the chuck body, a contacted surface with serrations extending circumferentially; a connecting member being inserted into a support groove that is formed in the contacted surface to extend radially and being incapable of coming out along the rotation axis (Jr); and a top jaw having a contact surface with serrations, fastened to the connecting member along the rotation axis (Jr) to come into close contact with the contacted surface, and being detachably attached to the master jaw to support the workpiece, the connecting member includes: a stopper extending along the support groove; and a key extending, with a width smaller than that of the stopper, in accordance with the stopper; a stopper extending along the support groove; and a key extending, with a width smaller than that of the stopper, in accordance with the stopper; the support groove includes: a first keyway into which the key is fitted with a tip of the key protruding from the contacted surface; and a stopper retainer configured to hold the stopper, the top jaw includes, on the contact surface, a second keyway into which the tip of the key is fitted, characterized in that a gap exists between the key and each of the first keyway and the second keyway, and a close contact between the contact surface and the contacted surface inclines or twists the key with respect to both the first keyway and the second keyway. a first keyway into which the key is fitted with a tip of the key protruding from the contacted surface; and a stopper retainer configured to hold the stopper, the top jaw includes, on the contact surface, a second keyway into which the tip of the key is fitted, characterized in that a gap exists between the key and each of the first keyway and the second keyway, and a close contact between the contact surface and the contacted surface inclines or twists the key with respect to both the first keyway and the second keyway.