To improve machining precision by detecting warping and positional deviation of a workpiece and compensating teaching coordinate of a robot arm.

A robot system is provided with a robot arm 2 consisting of two axes or more, a machining device 1 held at a tip shaft of the robot arm 2 to machine a workpiece 6, a teaching means for teaching a coordinate position for the machining device 1, an external device 8 capable of measuring a position of the machining device 1 for the workpiece 6, a coordinate compensation means 10 for compensating a coordinate of the machining device 1, and an interface means 9 having a plurality of communication procedures for communicating coordinate data with the external device 8 and capable of switching the communication procedures selectively depending on type of the external device 8 to be connected. Coordinate data of the machining device 1 is inputted from the external device 8 through the interface means 9, and the coordinate compensation means 10 compensates teaching coordinate of the machining device 1 taught by a teaching means based on the input value.