An engine for rotating a saw chain serving as a rotating tool includes an engine main body including a crank case and cylinder. A cooling fan for generating forced cooling air is attached to a protruding end of a crank shaft. An engine cover for covering a top of the cylinder is provided in the engine, and an air guiding rib extending in a direction of crossing the cooling air is provided in an inner circumferential surface of the engine cover. In the engine cover, opening portions for exhausting the cooling air outside are provided.