PURPOSE: To achieve the benefits of both loop scavenging and head scavenging simultaneously so as to improve scavenging performance by executing a second scavenging stroke via an intake valve arranged in a cylinder head and openable and closable by pressure differential besides executing loop scavenging as a first scavenging stroke.

CONSTITUTION: The lower surface of a piston 3 reciprocating in a cylinder 2 and a crankcase 6 constitute a pump which draws the combustion air via a reed valve 7 during the ascending stroke of the piston 3. Then, during the descending stroke of the piston 3, combustion air is delivered to a combustion chamber 1 via an intake passage 8 and discharged through a discharge slit 11 to scavenge exhaust gas (first scavenging stroke). In such a two-stroke engine, a flow passage 12 communicating with the intake passage 8 is formed in a cylinder head 4. A second scavenging stroke is executed by supplying the air of which the flow is adjusted by a throttle flap 14 via a flap valve 13 openable against a closure spring by the gas pressure differential between the flow passage 12 and the combustion chamber 1.