A control device for controlling an AC motor, the control device comprising a DC converter adapted to convert an AC voltage to a DC voltage; an intermediate circuit adapted to smooth the DC voltage input from the DC converter; an AC converter adapted to convert the DC voltage smoothed by the intermediate circuit to an AC voltage to be output to the AC motor; and an output controller (109; 309) adapted to control the AC converter and thus the operation of the AC motor, and a monitoring device (104; 304) adapted to monitor the AC motor as a safety unit independently of the control by the output controller and to provide instructions for decelerating or stopping the AC motor via an operation instruction unit to the output controller, the monitoring device comprising: a current detection unit (110, 210) adapted to acquire current data based on output current output to the AC motor; a voltage detector adapted to acquire voltage data based on an output voltage output to the AC motor; a speed computation unit adapted to calculate an estimated speed of the AC motor under a power running load or an estimated speed of the AC motor under a regeneration load based on the current data acquired by the current detection unit, the voltage data acquired by the voltage detector, a predetermined rated current, a preset rated voltage, a base frequency equivalent with a synchronous rotational speed, a rated rotational speed or rated slip speed and a number of poles; a determination unit adapted to determine based on the estimated speed of the AC motor calculated in the speed computation unit and a predetermined condition whether the estimated speed of the AC motor is within a predetermined range; and the operation instruction unit adapted to output an instruction for decelerating or stopping the AC motor to the output controller when the determination unit determines that the estimated speed of the AC motor is not within the predetermined range. a DC converter adapted to convert an AC voltage to a DC voltage; an intermediate circuit adapted to smooth the DC voltage input from the DC converter; an AC converter adapted to convert the DC voltage smoothed by the intermediate circuit to an AC voltage to be output to the AC motor; and an output controller (109; 309) adapted to control the AC converter and thus the operation of the AC motor, and a monitoring device (104; 304) adapted to monitor the AC motor as a safety unit independently of the control by the output controller and to provide instructions for decelerating or stopping the AC motor via an operation instruction unit to the output controller, the monitoring device comprising: a current detection unit (110, 210) adapted to acquire current data based on output current output to the AC motor; a voltage detector adapted to acquire voltage data based on an output voltage output to the AC motor; a speed computation unit adapted to calculate an estimated speed of the AC motor under a power running load or an estimated speed of the AC motor under a regeneration load based on the current data acquired by the current detection unit, the voltage data acquired by the voltage detector, a predetermined rated current, a preset rated voltage, a base frequency equivalent with a synchronous rotational speed, a rated rotational speed or rated slip speed and a number of poles; a determination unit adapted to determine based on the estimated speed of the AC motor calculated in the speed computation unit and a predetermined condition whether the estimated speed of the AC motor is within a predetermined range; and the operation instruction unit adapted to output an instruction for decelerating or stopping the AC motor to the output controller when the determination unit determines that the estimated speed of the AC motor is not within the predetermined range. a current detection unit (110, 210) adapted to acquire current data based on output current output to the AC motor; a voltage detector adapted to acquire voltage data based on an output voltage output to the AC motor; a speed computation unit adapted to calculate an estimated speed of the AC motor under a power running load or an estimated speed of the AC motor under a regeneration load based on the current data acquired by the current detection unit, the voltage data acquired by the voltage detector, a predetermined rated current, a preset rated voltage, a base frequency equivalent with a synchronous rotational speed, a rated rotational speed or rated slip speed and a number of poles; a determination unit adapted to determine based on the estimated speed of the AC motor calculated in the speed computation unit and a predetermined condition whether the estimated speed of the AC motor is within a predetermined range; and the operation instruction unit adapted to output an instruction for decelerating or stopping the AC motor to the output controller when the determination unit determines that the estimated speed of the AC motor is not within the predetermined range.