PURPOSE: To enable the exchange of a claw by a comparatively tiny device, by providing a claw storing magazine and a claw exchanger, in the automatic exchanger of the claw of a chuck holding a work according to the diameter of a holding part of the work.

CONSTITUTION: Sliding grooves 10 and 11 in a radiusward direction are formed on a machine tool, for example, a main body of a chuck to be mounted on the front edge of a main spindle 3 of an NC lathe 1, and a sliding piece component 12 and a sliding part 13a of a claw 13 are fitted in each of the grooves 10 and 11. The sliding piece component 12 and the claw 13 are connected with and disconnected from the main body 6 of the chuck by providing a claw connecting device 18 within the main body 6 of the chuck. A table 29 which is moved in a lengthwise direction by a cylinder 30 is provided on the uppr part of a machine bed and a cylinder 34 moving vertically is provided on the front end side of the table 29 in an upright state. A circular rotor 51 of a claw storing magazine 50 is fitted over a tube component 36a of the cylinder 34 and the claws 13 held by a large number of holders on the external circumference of the magazine 50 are inserted into and separated from a chuck 5 by a claw exchanger 65 consisting of a guide block 66 and a transfering device 67 of the claw.