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