An autonomous driving system mounted on a vehicle, includes: an information acquisition device configured to acquire driver operation information indicating an operation by a driver of the vehicle; and an autonomous driving control device configured to perform autonomous driving control of the vehicle. The autonomous driving control device performs: default lane setting processing that sets a default lane in which the vehicle travels; and vehicle travel control processing that controls the vehicle to travel in the default lane when there is no departure reason for departing from the default lane and to travel away from the default lane until there is no departure reason when there is the departure reason. In the default lane setting processing, the autonomous driving control device refers to the driver operation information to set the default lane according to the driver's intention.