To easily and efficiently reinforce a diving prevention beam, regardless of low cost, regarding a diving prevention device of a large-sized vehicle such as a truck.

The diving prevention beam 16 is located on a lower side of a frame 10 at an end part of the vehicle and is extended in a vehicle width direction. An upper part bracket 18 fixed to the frame 10 is extended downward to the front, a lower part bracket 20 is fastened to a front end with a bolt, and the upper and lower part brackets 18 and 20 are fastened with bolts and nuts. A reinforcing plate 28 is welded to a bottom surface of the diving prevention beam 16, and the lower part bracket 20 and the reinforcing plate 29 are welded to form a triangular closed section 30 inside the diving prevention beam 16. The reinforcing plate 29 is extended up to a bent end part of the diving prevention beam 16.