When a subject vehicle reaches a position PNO in a middle of an interflow zone, it is judged whether conditions for automatic interflow are satisfied. When it is judged that the conditions for automatic interflow are not satisfied at the position PNO, a notification for urging manual interflow by a driver is started. Otherwise, the notification for manual interflow is not started. In a case where the notification for manual interflow was not started, when the subject vehicle reaches the position PGU between a distal end of an interflow zone and the position PNO, the notification for manual interflow is started.