A monitoring device of a robot system, the robot system including: a robot including at least one joint axis (J1 to J6) and at least one servo motor (M1 to M 6) configured to drive the joint axis (J1 to J6); and a controller including a current sensor configured to detect a value of a current flowing through the servo motor (M1 to M 6) and configured to control the servo motor (M1 to M 6) which is configured to drive the joint axis (J1 to J6), the monitoring device comprising: a current/torque converting section configured to convert the value of the current flowing through the servo motor (M1 to M 6) which has been detected by the current sensor into a torque value; a driving torque estimating section configured to estimate at least a part of driving torque required to drive the servo motor (M1 to M 6); and a differential torque calculating section configured to calculate differential torque between the torque value obtained by conversion in the current/torque converting section and an estimated value of the driving torque; characterized by an external force converting section configured to convert the differential torque calculated by the differential torque calculating section into an external force applied to the robot; and a stop signal generating section configured to generate a stop signal for the robot based on a value of the external force obtained by conversion in the external force converting section, and to supply the stop signal to the controller, wherein the stop signal generating section includes a first collision determiner section configured to determine whether or not the value of the external force which has been obtained by conversion in the external force converting section is larger than a preset first threshold, and in a case where the first collision determiner section determines that the external force value is larger than the first threshold, the first collision determiner section is configured to determine that the robot has collided with an operator, to generate a first stop signal, and to output the first stop signal to the controller. the monitoring device comprising: a current/torque converting section configured to convert the value of the current flowing through the servo motor (M1 to M 6) which has been detected by the current sensor into a torque value; a driving torque estimating section configured to estimate at least a part of driving torque required to drive the servo motor (M1 to M 6); and a differential torque calculating section configured to calculate differential torque between the torque value obtained by conversion in the current/torque converting section and an estimated value of the driving torque; characterized by an external force converting section configured to convert the differential torque calculated by the differential torque calculating section into an external force applied to the robot; and a stop signal generating section configured to generate a stop signal for the robot based on a value of the external force obtained by conversion in the external force converting section, and to supply the stop signal to the controller, wherein the stop signal generating section includes a first collision determiner section configured to determine whether or not the value of the external force which has been obtained by conversion in the external force converting section is larger than a preset first threshold, and in a case where the first collision determiner section determines that the external force value is larger than the first threshold, the first collision determiner section is configured to determine that the robot has collided with an operator, to generate a first stop signal, and to output the first stop signal to the controller.