To provide a system and method for notifying a driver to the effect that another vehicle might be trying to enter, from a lane opposite to the driver, the same lane as the driver is about to enter. When an object vehicle and a second vehicle are on both sides of a target lane and at least one of the object vehicle and the second vehicle is about to move to the target lane, the method specifies the position of the object vehicle relative to the second vehicle. When an interval between the object vehicle and the second vehicle is decreasing, the method warns the driver of the object vehicle about the second vehicle. Furthermore, when the interval becomes shorter than a prescribed distance, the method guides the object vehicle so as to move away from the second vehicle.