Provided is an internal combustion engine having an oil circulating structure wherein oil used to lubricate valve operating members in the cylinder head drops onto an oil pan, and the oil accumulated therein is drawn up to the cylinder head again. Each on corners of the squire bottom face inside the cylinder head of the internal combustion engine, an oil dropping hole is provided to allow the oil used to lubricate valve operating members to flow thereinto. Also provided is an oil channel communicating with one or more of the oil dropping hole at the bottom thereof to guide the oil toward the driving gear transmitting the driving force of the engine.