A set of actuator units configured to be mounted in an airplane, wherein all the actuator units are configured to drive one control surface (0023,0025) of the airplane; each actuator unit comprises an actuator having a cylinder and a piston, a cylinder chamber (33a,33b) being arranged inside a body of the cylinder; each actuator unit has several operation modes including: a normal mode, which is set when the actuator units operate without any problem, and in which hydraulic fluid is supplied to the cylinder chamber (33a, 33b) so that the actuator is operated; a driven mode, which is set when a predetermined number or less of the actuator units are defective, and in which supply of hydraulic fluid to the cylinder chamber (33a, 33b) is stopped to cause the actuator to be driven by said control surface; and a back-up mode, which is set when a predetermined number or more of the actuator units are defective, and in which a servo-valve controlled by a back-up unit causes the actuator to be operated; each actuator unit further comprises: a controller attached to the actuator and configured to control a position of the piston, an actuator monitoring circuit configured to monitor the actuator in any one of the operation modes; wherein in each actuator unit, the actuator monitoring circuit is configured to output a notification signal (78s, 79s) when a calculated total moving distance driven by the piston up to a present time after a replacement of the actuator exceeds a predetermined threshold to notify excess over the threshold. all the actuator units are configured to drive one control surface (0023,0025) of the airplane; each actuator unit comprises an actuator having a cylinder and a piston, a cylinder chamber (33a,33b) being arranged inside a body of the cylinder; each actuator unit has several operation modes including: a normal mode, which is set when the actuator units operate without any problem, and in which hydraulic fluid is supplied to the cylinder chamber (33a, 33b) so that the actuator is operated; a driven mode, which is set when a predetermined number or less of the actuator units are defective, and in which supply of hydraulic fluid to the cylinder chamber (33a, 33b) is stopped to cause the actuator to be driven by said control surface; and a back-up mode, which is set when a predetermined number or more of the actuator units are defective, and in which a servo-valve controlled by a back-up unit causes the actuator to be operated; a normal mode, which is set when the actuator units operate without any problem, and in which hydraulic fluid is supplied to the cylinder chamber (33a, 33b) so that the actuator is operated; a driven mode, which is set when a predetermined number or less of the actuator units are defective, and in which supply of hydraulic fluid to the cylinder chamber (33a, 33b) is stopped to cause the actuator to be driven by said control surface; and a back-up mode, which is set when a predetermined number or more of the actuator units are defective, and in which a servo-valve controlled by a back-up unit causes the actuator to be operated; each actuator unit further comprises: a controller attached to the actuator and configured to control a position of the piston, an actuator monitoring circuit configured to monitor the actuator in any one of the operation modes; wherein in each actuator unit, the actuator monitoring circuit is configured to output a notification signal (78s, 79s) when a calculated total moving distance driven by the piston up to a present time after a replacement of the actuator exceeds a predetermined threshold to notify excess over the threshold. a controller attached to the actuator and configured to control a position of the piston, an actuator monitoring circuit configured to monitor the actuator in any one of the operation modes; wherein in each actuator unit, the actuator monitoring circuit is configured to output a notification signal (78s, 79s) when a calculated total moving distance driven by the piston up to a present time after a replacement of the actuator exceeds a predetermined threshold to notify excess over the threshold.