A monitoring device that monitors motions of a worker and a robot so that work is performed safely, the monitoring device comprising: an area setting unit, configured to set one of a worker protection area and a robot protection area as a protection area to secure safe work of the worker and the robot; at least one three-dimensional detection unit (4, 100), configured to specify positions of the worker and the robot in a three-dimensional space; a determination unit, configured to determine whether at least one of the worker and the robot intrudes into the set protection area on the basis of the positions of the worker and the robot specified by the three-dimensional detection unit (4, 100); a limiting unit, configured to limit the motion of the robot when the determination unit determines that at least one of the worker and the robot intrudes into the set protection area; and a switching unit, configured to transmit a switching signal to the area setting unit which is configured to, upon receipt of the switching signal, switch the set protection area from the set one of the worker protection area and the robot protection area to the other one, wherein the area setting unit is configured to set the worker protection area when the worker works in a common area in which a work area of the worker and a work area of the robot overlap and set the robot protection area when the robot works in the common area, the monitoring device being characterized in further comprising: a detection area setting unit, configured to set a detection area for detecting the position of the robot in the three-dimensional space by the three-dimensional detection unit (4, 100), wherein the determination unit determines whether the robot is in the detection area on the basis of the position of the robot specified by the three-dimensional detection unit (4, 100), and the switching unit is configured to transmit the switching signal when the determination unit determines that the robot is located in the detection area. an area setting unit, configured to set one of a worker protection area and a robot protection area as a protection area to secure safe work of the worker and the robot; at least one three-dimensional detection unit (4, 100), configured to specify positions of the worker and the robot in a three-dimensional space; a determination unit, configured to determine whether at least one of the worker and the robot intrudes into the set protection area on the basis of the positions of the worker and the robot specified by the three-dimensional detection unit (4, 100); a limiting unit, configured to limit the motion of the robot when the determination unit determines that at least one of the worker and the robot intrudes into the set protection area; and a switching unit, configured to transmit a switching signal to the area setting unit which is configured to, upon receipt of the switching signal, switch the set protection area from the set one of the worker protection area and the robot protection area to the other one, wherein the area setting unit is configured to set the worker protection area when the worker works in a common area in which a work area of the worker and a work area of the robot overlap and set the robot protection area when the robot works in the common area, the monitoring device being characterized in further comprising: a detection area setting unit, configured to set a detection area for detecting the position of the robot in the three-dimensional space by the three-dimensional detection unit (4, 100), wherein the determination unit determines whether the robot is in the detection area on the basis of the position of the robot specified by the three-dimensional detection unit (4, 100), and the switching unit is configured to transmit the switching signal when the determination unit determines that the robot is located in the detection area. a detection area setting unit, configured to set a detection area for detecting the position of the robot in the three-dimensional space by the three-dimensional detection unit (4, 100), wherein the determination unit determines whether the robot is in the detection area on the basis of the position of the robot specified by the three-dimensional detection unit (4, 100), and the switching unit is configured to transmit the switching signal when the determination unit determines that the robot is located in the detection area.