The present invention is provided for sufficiently ensuring a heat balance in the inside of a bonnet. In a tractor in which a radiator including an outside-air suction fan is arranged frontward of an engine, the radiator and the outside-air suction fan are covered with a bonnet, and the outside air is sucked by the outside-air suction fan through a front grille portion formed in the bonnet thus cooling the radiator, a radiator cooling air discharge opening portion is formed in a side wall portion of the bonnet, and the radiator cooling air discharge opening portion opens in the rearward direction.