A monitoring device of a robot system including: a current sensor detecting a value of a current flowing through the servo motor; a current/torque converting the value of the current flowing through the servo motor which is detected by the current sensor into a torque value; a driving torque estimating section estimating at least a part of driving torque required to drive the servo motor; a differential torque calculating differential torque between the torque value obtained by conversion in the current/torque converting section and an estimated value of the driving torque; an external force converting the differential torque calculated by the differential torque calculating section into an external force applied to the robot; and a stop signal generating section which generates to stop the robot based on a value of the external force obtained by conversion in the external force converting section, and supplies the stop signal to the controller.