unidirectional valve 70 as a second oil feeding means the unidirectional valve 70 is composed of a valve hole 72 piercing through a lower portion of the crankcase 14 and a valve plate 74 for opening the valve hole 72 when the crank room 16 becomes positive in pressure and closing the valve hole 72 when the crank room 16 becomes negative in pressure in accordance with the up and down movements of the piston 24 in fig2 a breather pipe 80 is provided onto the top of the cylinder block 12 the breather pipe 80 has an end communicated with the inside of the valve gear room 34 via an opening 82 and the other end connected to the air cleaner 2 the valve gear room 34 comprises oil returning channels 84 and 84 u2032 each of these has an end opened to the valve gear room 34 and the other end opened to the oil sump 18 in such constitution as shown in fig1 being positioned with the valve gear room 34 up in other words in an upright state the engine 1 holds proper amounts of lubricating oil in the crank room 16 oil sump 18 and valve gear room 34 in the case where the piston 24 is not in up and down motion when.