An oil storing device includes a body portion. The body portion is formed at its one end with an engaging portion having an oil supply port for supplying engine oil and an oil return port for introducing the engine oil. The engaging portion is detachably connected to a connecting portion of a crankcase. Oil storing chambers for storing the engine oil are formed in the body portion so as to be connected to the oil supply port and the oil return port. When the engaging portion is engaged with the connecting portion of the crankcase, the oil supply port is connected to an inlet port leading to an oil pump, and the oil return port is connected to an outlet port to the crankcase.