A vertical internal combustion engine E is incorporated into an outboard motor or the like. The vertical internal combustion engine E includes a crankshaft 18 enclosed in a crank chamber 20, and a belt-drive transmission mechanism 50 including a rubber belt 53 for transmitting power of the crankshaft 18 to a camshaft 24, and held in a belt chamber 63. The belt chamber 63 is isolated from the crank chamber 20 by a lower case 61 of a transmission case 60 holding the belt-drive transmission mechanism 50 therein such that the flow of an oil-containing gas from the crank chamber 20 deviates from a direction toward the belt 53. The belt 53 is lubricated with oil injected concentratedly into the belt chamber 63 through jet nozzles 80 disposed in the belt chamber 63. Thus the excessive exposure of the belt 53 to the oil-containing gas from the crank chamber can be avoided, the life of the belt can be extended and the belt is lubricated properly.