An engine including first scavenging passages (13) for fluidly connecting between a combustion chamber (1a) and a crank chamber (2a) through bearings (81) for the crankshaft (8), second scavenging passages (14) fluidly connecting directly between the combustion chamber (1a) and the crank chamber (2a), a suction chamber (72) formed in a side face of a piston (7). An air-fuel mixture (M) is sucked into the suction chamber (72) and an air (A) is introduced into the crank chamber (2a). During the intake stroke, the air-fuel mixture (M) from the air-fuel mixture passage (10) is introduced into the first scavenging passages (13) through the suction chambers (72), and the air (A) is introduced into the crank chamber (2a). During the scavenging stroke, before the air-fuel mixture (M) in the first scavenging passages (13) is introduced into the combustion chamber (1a), introduction of the air (A) in the crank chamber (2a) into the combustion chamber (1a) through the second scavenging passages (14) starts.