To improve the bending strength of an underrun protector provided on a truck and the like while avoiding mass increase as far as possible.

In the underrun protector disposed in a vehicular width direction on at least one end side in a fore-and-aft direction of a vehicle and mounted/fixed to mounting portions (mounting plates 12) on both sides in the vehicular width direction provided on a vehicle body side, an extension member (an extension plate 2) extending from the mounting portion to the inside in the vehicular width direction along an mounting surface of the underrun protector mounted on the mounting portion. The extension member (the extension plate 2) is preferably extended from the mounting portions (the mounting plates 12) side on both sides to the inside to be integrated with each other. The bending strength of the underrun protector is improved by the extension member, and the occurrence of large bending due to the concentration of stress in the underrun protector neat the mounting portion upon collision is prevented.