PURPOSE: To perform automatic change in an upper jaw at will, by using a clamping mechanism built-in chuck jaw consisting of a lower jaw and the upper jaw clamped by the former.

CONSTITUTION: A pair of upper and lower opening concave parts 31b and 31c are formed in an upper jaw setting parts 31a of a lower jaw 31, while an upper jaw holding shaft 31d pierces through the center of these concave parts 31b and 31c free of vertical motion. The concave part 31b side is energized upward by dint of a spring 31e, and a columnar tilt clamping piece 31g is formed at the side of the concave part 31c. In contrast with this, at the upper part of an upper jaw 32, projecting part 32b forming a tilt concave groove 32a correspondent to the clamping piece 31g is projectingly formed in conformity with an inner circumferential form of the concave part 31c of the lower jaw 31, and the clamping piece 31g is relatively slidden in an inclined direction in the state that the clamping piece 31g is made to go down to some extent against the energizing force of the spring 31e whereby engagement or disengagement of the upper jaw 32 to or from the lower jaw 31 takes place. Therefore, automatic change in a chuck is easily performable by using an automatic upper jaw change device alone.