To provide an air resistance reducing structure of a flat body vehicle with an owning, wherein an air resistance reducing device makes easy adjustment to provide a maximum effect in accordance with the load state of the flat body vehicle the hood, i.e., the state of the hood, and any effect of the state of the hood is eliminated even if the hood is removed.

A flat plate bar structure 4 is made in such a manner that vertical flat plate bard 3 roughly vertically intersect the vicinity of both ends of a horizontal flat plate bar 2 roughly parallel to the upper end member 1c of a guard frame 1, and the flat plate bar structure 4 is connected to the guard frame 1 by a connecting bolt 1b or the like through an oblong hole 5 provided in the vertical flat plate bar 3 of the flat plate bar structure 4. The installing height of the flat plate bar structure 4 is adjusted according to the height of a closed plane formed by a hood on a cap. Thus, an optimal air resistance reducing effect is provided in according with the state of a hood. In the folded state of the hood, the height position of the flat plate bar structure 4 is adjusted until the horizontal flat plate bar 2 is brought into contact with the upper end member 1c of the guard frame 1. Thus, the flat plate bar structure 4 is prevented from becoming a substantial air resistance generating source, and also prevented from becoming an obstacle when a long article is rested on the guard frame 1.