To provide an air supply and exhaust device for a two-cycle engine allowing effective combustion of a mixture while effectively suppressing blow-by of an unburnt mixture to an exhaust port.

In the two-cycle engine provided with the exhaust port, scavenging ports and an intake port opening/closing valves following the vertical movement of a piston 1, and a fuel injector, a mixture retaining cavity 15 lurking on the scavenging port 7 side is formed at a part of a head crown face of the piston 1. Fuel supplied from the fuel injector 11 is thereby mixed with air from the intake port 10, spouted into the mixture retaining cavity 15 and effectively separated from a scavenging air flow from the scavenging ports 6, 7. A possibility of the mixture unreasonably leaking out of the exhaust port 5 is thereby eliminated to suppress the degradation of combustion efficiency and output.