PURPOSE: To exhaust unburned gas and improve scavenging efficiency, by furnishing an air passage port at the crank case of a two-cycle engine with a non- ignition cycle, in which a switch valve is arranged to suck in the air in the non-ignition cycle.

CONSTITUTION: Mixed gas is fed through a lead valve 18 from a carburettor 19 into the crank case of a two-cycle engine. The engine is a two-cycle engine with a non-ignition cycle, where the ignition-plug 11 of the engine ignites only in one cycle of the two crank shaft rotations, and does not ignite in the other cycle. The crank case 2 has an air passage 22 to connect to the atmosphere, in which a control valve 28 and a switch valve 23 are furnished. Since the switch valve 23 is engaged with the crank shaft gear 26 and opened or closed by a gear 25 which rotates at a half speed, only the outer air is inhaled in the non-ignition cycle into the crank case 2 with no suction or exhaust of mixed gas, and scavenging is performed thoroughly.