An inner circumferential surface of a chuck body (2) has three fitting recesses (22) curved as viewed from a rotational axis (C1). An outward elastic deformation that occurs, through gripping of a work (W1) with top jaws (5), at portions of a plunger (3) which correspond to slide holes (31a) causes the portions deformed by the elastic deformation to slide on the respective recessed portions (22) so that the plunger (3) is moved to cause a symmetry axis (Y1) of the slide hole (31a) to coincide with a symmetry axis (Y2) of a guide groove (25) of the chuck body (2).