A processor operates to: acquire first information regarding a surrounding travel environment including a travel lane in which a subject vehicle travels, on the basis of detection information from an onboard sensor; refer to map information stored in a storage device to acquire second information regarding lanes of a road; determine whether or not a travel road to which the travel lane belongs is a predetermined specific road, on the basis of the first information; and when a determination is made that the travel road is the specific road, composite the first information and the second information to generate travel environment information.