PURPOSE: To provide an internal combustion engine which is switcheable between two-cycle operation and four-cycle operation and draws the advantages of a two-cycle engine and a four-cycle engine, reduce weight, improve performance, improve exhaust gas, reduce fuel consumption, and increase a brake force.

CONSTITUTION: An internal combustion engine comprises a hydraulically-driven fuel injection valve 5; a hydraulically-driven intake valve 6; a hydraulically- driven exhaust valve 7; and an exhaust turbo supercharger 10 for scavenging. The fuel injection timing of the hydraulically-driven fuel injection valve 5, and the opening closing timings of the hydraulically-driven injection valve 6 and the hydraulically-driven valve 7 are controlled by a controller 11. Further, during high load running, an engine is switched to two-cycle operation and during low load running, it is swithceable to a four-cycle operation in a way that the number of working times of the hydraulically-driven injection valve 5 and hydraulically-driven intake exhaust valves 6 and 7 is controlled to one working time per one full rotation or half rotation of an engine.