The possibility of a lane change of a vehicle is determined by using a first inter-vehicle distance that serves as a target for the lane change of the vehicle on an adjacent lane adjacent to a lane on which the vehicle is traveling. When the lane change is determined not to be executable, the presence or absence of a possibility that the first inter-vehicle distance will extend to a length that allows for the lane change is determined by using a second inter-vehicle distance positioned in front of or behind the first inter-vehicle distance. When it is determined that there is the possibility, waiting is determined to be necessary, and when it is determined that there is no such possibility, the waiting is determined to be unnecessary.