Abstract: A horizontal type Diesel engine wherein a gear case is juxtaposed to a crankcase axially of a crank shaft, said crankcase horizontally receiving a cylinder therein and a radiator and a fuel tank are juxtaposed on top of the crankcase. The Diesel engine is characterized by a breather disposed on the upper wall of the gear case substantially at the middle between the front and rear ends thereof and an oil partition wall opposed to the breather, the arrangement being such that even if the vehicle on which this engine is mounted is tilted, the lubricating oil contained in the crankcase and gear case will not leak. Further, a governor shaft vertically pivotally supported by the gear case has its one end projecting into the breather so as to prevent the oil from leaking into the outside along the end of the governor shaft.