PURPOSE: To obtain static pressure balances in axial direction and tangential direction by varying sector surface of an envelope adjacent to engagement point until it becomes an obstacle of a permanent high pressure zone while obtaining axial balancing of a driving gear by varying value of surface of the permanent high pressure zone.

CONSTITUTION: Main body of the apparatus is composed of a driving gear 9 and a driven gear 10. When inter balancing is ensured by a hydraulic coil system and an engagement without clearance, inter sealing is realized by two side plates 21, 22, and a soft system comprising an envelope 36 provided with static pressure compensation generated by sectors 60', 60". In this invention, balancing in axial direction of the driving gear 9 is effected by varying value of the surface of a permanent high pressure zone 34. The value variation causes an application of the high-pressure on one of the side plates and does not cause influence on the other side plate. In addition, in order to obtain tangential balancing of the driving gear 9, sector surface of the envelope 36 adjacent to the engagement point 3 is varied until it becomes an obstacle of the zone 34.