The present invention allows an existing single-sequentialized navigation main unit to be virtually multi-sequentialized with simple structure. An intervention device 3 receives from a front monitor 4 and a rear monitor 5 acquisition requests which request various kinds of screens from a navigation main unit 2. In response to the acquisition requests, the intervention device 3 performs a mediating process or an adjusting process for various kinds of screens which are received from the navigation main unit 2 and are scheduled to be output to the front monitor 4 and the rear monitor 5. Therefore, even if the intervention device 3 receives a plurality of acquisition requests from the front monitor 4 and the rear monitor 5, the intervention device 3 can sequentially send the acquisition requests one by one to the navigation main unit 2. Accordingly, this allows the navigation main unit 2 to be virtually multi-sequentialized.