Abstract: There is provided an under-run protector which is light-weighted and has sufficient strength, owing to a change in a plate thickness of its portion optimum for strength improvement. In an under-run protector which includes a hollow material having a rectangular cross section and mounted on a vehicle body frame via a mounting bracket, a plate thickness of a substantially horizontal surfaces forming the hollow material on a vehicle body frame side with respect to a longitudinal-direction center line of the hollow material is larger than a plate thickness of a substantially horizontal surfaces opposite the vehicle body frame with respect to the longitudinal-direction center line of the hollow material.