PURPOSE: To improve durability and efficiency of a pump by introducing corresponding toothed groove pressure to the back of lip seal at low pressure side.

CONSTITUTION: Shallow grooves 19, 20 and deep grooves 21 through 24 are formed inside a cover 17, a series of first lip seal 36 and first backup 37 are fitted in the shallow grooves 19, 20 and the deep grooves 23, 24, and further second lip seals 38, 39 and second backups 40, 41 are fitted in the deep grooves 21, 22. Then, the cover 17 is junctioned to the side of a body 11 to form loading areas M1 through M4. Since a corresponding toothed groove pressure is introduced to the back of second lip seals 38, 39 by way of holes 44, 45 and loading area M4, the corresponding portion will never be loaded excessively. Therefore, a side plate will not be worn out one-sidedly or a brake will not be applied against input torque, thus improving durability and efficiency of a pump remarkably.