An oil cooling structure of an engine is provided that is capable of enhancing cooling performance of oil while preventing an increase in number of components or an increase in required space from being involved as much as possible. In an oil cooling structure of an engine in which a transmission case is attached to one end wall of an engine body, and a transmission mechanism for an accessory drive is provided in the transmission case, the transmission case is formed into a potlid-shaped body having a plurality of rib walls along a front-rear direction, and is attached to an engine front wall with an engine cooling fan, and case inner chamber portions partitioned by the rib walls in the transmission case are configured as a transporting passage of oil circulated by an oil pump.