passage is required for its drainage thus the cost can be lowered fig3 and 4 show a second embodiment and in particular arrangement of a parking gear and a parking brake in this second embodiment automatic transmission casing 11 serves as the rotary member enclosure and a parking gear 71 as the rotary member which is arranged on the output shaft 81 rotatably relative to the automatic transmission casing 11 a plurality of gear teeth 71b and spaces 71c are alternately formed around the outer circumference of the parking gear 71 a parking brake 77 is pivotally supported on the automatic transmission casing 11 and carries a pawl 78 which is adapted to be selectively engaged within the gear spaces 71c when the driver operates the not shown shift lever to select the parking range the parking brake 77 is pivoted to bring the the pawl 78 into mating engagement within a space 71c to thereby lock the output shaft 81 the parking gear 71 is fixed on the output shaft 81 so that it is turned clockwise in the drawing when the vehicle runs in reverse conventionally the oil having lubricated and cooled the individual rotary members of the automatic transmission adheres to the outer circumference 71a of the parking gear 71 since the oil becomes more viscous and less fluid at the lower temperature however it resists the centrifugal force and remains on the outer circumference 71a as a result the amount of oil recovered in the oil pan as the oil reservoir is reduced to lower its level thus oil separating means are formed on the inner circumference 11a of the automatic transmission casing 11 facing the outer circumference 71a of the parking gear 71 and located upstream.