To enable reliable detection of a vehicle rear end part by means of a millimeter wave radar or the like from a following vehicle, and to enable suppression of loading efficiency degradation.

An air resistance reducing device of a vehicle, mounted on a rear end surface of a van body 4 of the vehicle, includes: peripheral edge protrusion parts 50, 60, 70 which are disposed on the peripheral edge part of the rear end surface and are protruded to the vehicle rear side; and a flat surface part 40 which is joined with protrusion ends of the peripheral edge protrusion parts 50, 60, 70, is separated from the rear end surface to the rear side and is formed in parallel or substantially in parallel to the rear end surface; wherein there are provided guide surfaces 51, 61 which are obliquely protruded toward the vehicle rear side and the neighborhood of the vehicle center from the peripheral edge part to guide an air current, on the peripheral edge protrusion parts 50, 60 disposed on the both side edge parts and upper edge part of the rear edge surface in the peripheral edge part.