To provide an air resistance reducing device of a vehicle capable of reducing air resistance of a cargo vehicle in a loaded state of a loading box, as well as a non-loaded state.

A reducing device 21 is attached to a cabin roof 15 of a tractor 11 capable of towing a trailer 13 loaded with a loading box 12. The reducing device 21 is equipped with a plate 22 rotatably supported to the cabin roof 15 by a rotation supporting part 25, and a cylinder 23 arranged at the back of the plate 22. The cylinder 23 is designed to make the height of the plate 22 to a first predetermined height H1 for reducing air resistance under a condition that the tractor 11 drives while towing the trailer 13 by pushing a piston rod 23a. When the trailer 13 is not towed and only the tractor 11 drives, the cylinder 23 is designed to make the height of the plate 22 to a second predetermined length H2 for restraining exfoliation of air flow generated in the cabin roof 15 and reducing the air resistance by towing the piston rod 23a.