PURPOSE: To increase the response and the reliability when the 2-cycle operation or the 4-cycle operation is converted, by maintaining the condition to block the suction port of a slide valve constantly with a spring, or opening the suction port by applying an air pressure to the end surface to drive the slide valve.

CONSTITUTION: In a low speed and a high load condition of this engine, for example, a controller 8 opens an electromagnetic valve 42 so as to open an air chamber 33 to the outside as well as feeding the compressed air from a pressure tank 41 to an air chamber 32. As a result, two slide valves 3 are moved in the direction to contract the air chamber 33 against the extension force of a spring 34 by the pressure in the air chamber 32. And when the slide valves 3 are stopped, suction ports 12 and windows 31 are made coincident, and thereby, the suction air compressed by a turbocharger 7 is fed from the suction ports 12 to a combustion chamber. Consequently, the engine now operates as a uniflow type 2-cycle engine.