An oil reservoir chamber, a crank chamber and a valve-operating chamber are provided in an engine body. The oil reservoir chamber and the crank chamber are in communication with each other through a through-hole, and the crank chamber and the valve-operating chamber are in communication with the each other through a one-way valve which is opened upon rising of the pressure in the crank chamber. The valve-operating chamber and the oil reservoir chamber are in communication with each other through an orifice. By utilizing a pressure pulsation in the crank chamber, an oil mist produced in the oil reservoir chamber is circulated from the oil reservoir chamber to the crank chamber, the valve-operating chamber and the oil reservoir chamber. Thus, in any operational attitude of an engine, the lubricating oil can be circulated without use of special oil pump.