A vertical internal combustion engine E has a crankshaft 18 enclosed in a crank chamber 20, a belt-drive transmission mechanism 50 held in a belt chamber 63 and including a rubber belt 53 for transmitting power of the crankshaft 18 to a camshaft 24 in a valve train 23, and a transmission case 60 defining the belt chamber 63. The belt chamber 63 communicates with the crank chamber 20 by way of vent holes 70 and 71, the transmission case 60 is provided with an internal wall Wi disposed between the vent holes 70 and 71 and the belt 53 in the belt chamber 63 to deflect the flow of an oil-containing gas flowing from the crank chamber 20 into the belt chamber 63 such that the oil-containing gas flows in directions deviating from a direction toward the belt 53. The belt 53 is lubricated with oil in the oil-containing gas. The belt 53 is prevented from being excessively exposed to the oil-containing gas from the crank chamber 20 to extend its life.