An autonomous driving system acquires information concerning an empty space situation in an adjacent lane, and information concerning an entry frequency with which other vehicles enter into an own lane from the adjacent lane, when an own vehicle travels on a road having a plurality of lanes. The autonomous driving system determines whether or not to select the adjacent lane as an own vehicle travel lane in accordance with the empty space situation in the adjacent lane and the entry frequency. However, when the entry frequency is a threshold value or more, determination not to select the adjacent lane as the own vehicle travel lane is kept irrespective of the empty space situation in the adjacent lane. The autonomous driving system performs lane change to the adjacent lane autonomously when the adjacent lane is selected as the own vehicle travel lane.