To avoid the deviation of lubricating oil stored in a bottom portion of a casing in the axial direction of transmission shafts when an automobile mounted with a transmission is suddenly accelerated.

On a middle between shafts 12, 13 of the casing 10 in the axial direction, there are provided an oil stopper plate 41 supported between the bottom portion of the casing 10 and an oil separator 31 turnably in the direction perpendicular to the axial direction, and an inertial mass 46 for turning the oil stopper plate 41 when a speed of acceleration due to sudden acceleration is applied to the shafts 12, 13 in the axial direction. When the speed of acceleration due to sudden acceleration is applied to the shafts 12, 13 in the axil direction, the oil stopper plate 41 is turned with inertia working on the inertial mass 46 to partition a space between the bottom portion of the casing 10 and the oil separator 31, thus preventing the deviation of lubricating oil stored therein in the axial direction of the shafts 12, 13.