Abstract: A highly efficient spark ignition two-cycle internal combustion engine is provided which assures stable and excellent combustion over a wider operation region of the engine. The spark ignition two-cycle internal combustion engine includes a sub-combustion chamber, and a pressure accumulating chamber which is filled with compressed air. A switching valve is arranged for communicating the pressure accumulating chamber with the sub-combustion chamber. A fuel injecting device injects fuel into the pressure accumulating chamber. A control device controls the operations of the switching valve and the fuel injecting device.