A structure of an underrun protection bound to a plurality of brackets disposed at the front or rear portion of a frame of a vehicle is disclosed. The structure has a hollow having a closed section defined by a distal face of a longitudinal direction of the vehicle, a proximal face of the longitudinal direction, an upper face, and a lower face. A plurality of lightening holes are formed in line along an extending direction of the underrun protection on at least one of the distal face and the proximal face. The plurality of lightening holes have diameters in vertical direction set larger as departing farther from binder parts each at which the underrun protection is bound to one of the plurality of brackets.