The robot system includes a robot having an arm which is provided, on its tip, with two claws which are driven by a servo motor. According to this robot system, a relative position of the two claws is detected when the two claws apply a load of a predetermined force to a welded part welded to a workpiece. The deviation between the detected relative position and a predetermined reference relative position is calculated. The workpiece is determined to be of a good quality or poor quality by comparing the calculated deviation to a predetermined threshold value.