Abstract: A water-cooled type internal combustion engine for a farming motor vehicle has a muffler arranged within the space on the lateral side of an engine body disposed in the fore and rear direction of the farming motor vehicle. An exhaust gas dilution pipe is externally fitted at its upper end portion to an outlet pipe of the muffler so as to provide a cooling air passage gap therebetween. The lower end portion of the dilution pipe is extended below the projected end portion of the outlet pipe. A portion of the cooling air delivered by the operation of a radiator fan so as to pass through a radiator is guided by means of a muffler cover so as to be forcedly fed into the dilution pipe through the cooling air passage gap in order to facilitate the dilution of the exhaust gas.