Abstract: An oil supply passage is provided for connecting an oil chamber with a valve chamber, an oil feed passage for connecting a valve chamber with a crank chamber, and an oil return passage for connecting the crank chamber with the oil chamber. The oil return passage is connected with the crank chamber via a plurality of return vents formed on an inner surface of the crank chamber at approximately equal intervals. Providing a bypass passage, one end of the bypass passage opens adjacent to a ceiling of the valve chamber. Another end of the bypass passage opens into a portion facing the restrictor formed adjacent to an outlet of the oil feed passage. Accordingly, the crank chamber and the valve chamber are lubricated by pressure fluctuations of the crank chamber according to the reciprocation of a piston. Thus, it is possible to lubricate the engine even when the engine is tilted in any direction.