A four-stroke engine configured 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, the four-stroke engine comprising: an intake valve and an exhaust valve configured to open and close a combustion chamber; a rocker chamber configured to accommodate the intake valve and the exhaust valve; a cam to drive the intake valve and the exhaust valve; cam driving parts (31, 33) to drive the cam; a driving chamber configured to accommodate the cam driving parts (31, 33); a tank to accumulate oil therein; and a gas-liquid separator configured to separate blowby gas from oil used for lubrication, wherein: the cam driving parts (31, 33) move with rotation of the crankshaft; the driving chamber and the rocker chamber are connected to one another via a push rod guide; the rocker chamber is connected to the crank chamber and the gas-liquid separator; the oil accumulated in the 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; the communicating passage is connected to a lower part of the crank chamber; the return passage is connected to an upper part of the driving chamber; 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 a top dead center; and oil flows through the crank chamber, the communicating passage, the driving chamber, the return passage and the crank chamber in this order to form one oil circulating route. an intake valve and an exhaust valve configured to open and close a combustion chamber; a rocker chamber configured to accommodate the intake valve and the exhaust valve; a cam to drive the intake valve and the exhaust valve; cam driving parts (31, 33) to drive the cam; a driving chamber configured to accommodate the cam driving parts (31, 33); a tank to accumulate oil therein; and a gas-liquid separator configured to separate blowby gas from oil used for lubrication, wherein: the cam driving parts (31, 33) move with rotation of the crankshaft; the driving chamber and the rocker chamber are connected to one another via a push rod guide; the rocker chamber is connected to the crank chamber and the gas-liquid separator; the oil accumulated in the 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; the communicating passage is connected to a lower part of the crank chamber; the return passage is connected to an upper part of the driving chamber; 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 a top dead center; and oil flows through the crank chamber, the communicating passage, the driving chamber, the return passage and the crank chamber in this order to form one oil circulating route. the cam driving parts (31, 33) move with rotation of the crankshaft; the driving chamber and the rocker chamber are connected to one another via a push rod guide; the rocker chamber is connected to the crank chamber and the gas-liquid separator; the oil accumulated in the 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; the communicating passage is connected to a lower part of the crank chamber; the return passage is connected to an upper part of the driving chamber; 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 a top dead center; and oil flows through the crank chamber, the communicating passage, the driving chamber, the return passage and the crank chamber in this order to form one oil circulating route.