PURPOSE: To surely prevent exit of oil from an oil pan, by providing a return opening, which allows no oil to pass therethrough to a return oil passage side, at substantially central position of an upper portion of the oil pan.

CONSTITUTION: A fixed amount of oil is always accommodated in an oil pan 39 with the aid of a level gauge. Under such an oil accommodated condition, when a casing 14 is thrown down sideways in such a manner that for example, its front side surface is converted to a mount surface, an oil level in the oil pan 39 becomes a level as depected by 47A, and accordingly, oil in the oil pan 39 is held in the oil pan 39 without being discharged from a return opening 45 to return oil passages 43 and 44 side.