A four-stroke engine is provided to lubricate driving parts including a crankshaft and valve operating members while circulating oil, using pressure fluctuation in a crank chamber, the pressure fluctuation being caused by reciprocating motion of a piston. A cam driving parts move with rotation of the crankshaft; a driving chamber and a rocker chamber are connected to one another; the rocker chamber is connected to the crank chamber and a gas-liquid separator; the oil accumulated in a tank is sucked up into the crank chamber and circulated through each part of the four-stroke engine; the crank chamber and the driving chamber are connected to one another via a communicating passage and a return passage; and the crank chamber and the driving chamber connected to one another via the return passage communicate with one another only when the piston is located in a vicinity of the top dead center.