A robot system capable of performing automatic updating of data inherent to a robot mechanism section or a mechanical unit thereof when changing the robot mechanism section or the mechanical unit. After changing the robot mechanism section or the mechanical unit, data of identifiers are read by a robot control section from nonvolatile memories associated with encoders in the robot mechanism section or the mechanical unit automatically upon turning-on of a power supply on or a manual operation. If it is determined that a kind of the robot is changed, the data indicating the kind of robot is rewritten in the robot control section. If it is required to change an algorithm for forward/inverse transformation for calculation of a robot locus, the algorithm is changed. If a kind of the robot is not changed, it is determined whether individuality of the whole robot mechanism section or the mechanical unit is changed or not. If there is a change in the individuality, the data inherent to the individuality is read from the nonvolatile memories to update the corresponding data in the robot control section.