A lubrication structure is provided for supplying lubricating oil to a transmission of an engine. The engine includes a mission holder which includes a power transmission mechanism between a main shaft and a countershaft as a unit and is removably mounted in a mission chamber, and a clutch chamber having a clutch mechanism is provided adjacent the mission chamber for transmitting rotation of a crankshaft to the transmission. Two oil introduction ports are provided on the mission holder, which is between the clutch chamber and the mission chamber. The two oil introduction ports enable the two chambers to communicate with each other. Each of the oil introduction ports has an opening provided therein which opens upwardly on a clutch chamber side of the mission holder.