In an automatic chuck jaw change system, a combined machining lathe capable of positioning with high precision and a robot with lower positioning precision than the combined machining lathe cooperate to automatically change a chuck jaw. The system includes a robot disposed in front of the combined machining lathe, and the robot can be selectively mounted on a front end of an arm thereof with a work hand for operating a work, a chuck jaw, and a nut runner hand for operating a drive mechanism of the chuck. The robot cooperates with a tool for jaw mounted on a tool spindle of the combined machining lathe to automatically change a chuck jaw of a main spindle.