PURPOSE: To drastically improve the efficiency of an engine by generating a high torque in the low speed operation and carrying out the operation with high fuel consumption in the high speed operation by selectively switching three operation conditions of turbocharger operation, two and four cycle operations according to the operation state of the engine.

CONSTITUTION: In the low speed high load state, a controller 8 operates an engine in two-cycle operation, and suspends the operation of an intake valve driving device 51 and holds an intake valve 5 in closed state. further, the controller 8 controls a exhaust valve driving device 61 and opens an exhaust valve 6 in each revolution. When a slide valve 3 stops, an intake port 12 and a window part 31 coincide, and the intake air compressed by a turbocharger 7 is supplied into a combustion chamber from the intake port 12. Accordingly, this engine operates as uniflow type two-cycle engine. Then, in the high speed operation, the ordinary four-cycle operation is performed by the intake valve driving device 51 and the exhaust valve driving device 61.