To provide a small lightweight high output engine.

In this engine, exhaust ports are newly provided on an engine block upper part side surface both side (code 3 of a drawing), the two exhaust ports which become needless by that are newly converted to intake ports, the converted two intake ports are added to original two intake ports and total four intake ports are used to feed double air fuel mixture gas to a combustion chamber at one time without changing a model of an engine main body. Consequently, output of the engine can be doubled without enlarging the engine. Since just the exhaust ports are newly provided on the engine block upper part side surface both side and new intake ports are converted from the exhaust port, the engine main body is not modified greatly and development cost can be kept low with obtaining doubled output. The engine can be miniaturized, lightened and output can be increased with reducing development cost.