FIGS. 16A to 16C schematically illustrate, in the order of time passage, an example of displaying prior notice at the first display device 3, that is, the front windshield 15 in a case that lane change is performed from a cruising lane 51 to an opposing lane 52. FIGS. 17A to 17C schematically illustrate, in the same order of time passage, displaying of prior notice at the second display device 4, that is, the monitor device 24 in the above case.