An object of the present invention is to enhance the reliability of an autonomous driving system. The autonomous driving system includes: a higher-level control device 1 that outputs a control target value of an actuator group based on an action plan of a vehicle; and a lower-level control device 2 that controls the actuator group of the vehicle based on a command from the higher-level control device 1. The lower-level control device 2 holds the control target value of the vehicle provided by the higher-level control device 1 over a specific period. When the higher-level control device 1 does not satisfy a desired function, the lower-level control device 2 is configured to be controlled based on the held control target value. The action plan is followed by determining and correcting a difference between an actual action value and the control target value of the vehicle.