A hydraulic device at least comprising: a pair of helical gears (20, 23) which each have a rotating shaft (21,24) provided to extend outward from both end surfaces thereof, and whose tooth portions mesh with each other, the pair of helical gears having a tooth profile including an arc portion at a tooth tip and a tooth root, and forming a continuous line of contact from one end portion to the other end portion in a face width direction at a meshing portion; a body open at both ends and having a hydraulic chamber therein in which the pair of gears (20, 23) are contained in a state of meshing with each other, the hydraulic chamber having an arc-shaped inner peripheral surface with which outer surfaces of the tooth tips of the gears are in sliding contact; a pair of bearing members (40, 44) which are respectively disposed on both sides of the gears in the hydraulic chamber of the body and which support the rotating shafts (21,24) of the gears so that the rotating shafts are rotatable; a pair of cover plates (7, 8) which are respectively liquid-tightly fixed to both end surfaces of the body to seal the hydraulic chamber, the hydraulic chamber having a low-pressure side defined on one side of the meshing portion of the pair of gears and a high-pressure side defined at the other side thereof; and the body having a flow path which opens into an inner surface of the low pressure side of the hydraulic chamber and a flow path which opens into the inner surface of the high pressure side of the hydraulic chamber, characterised in that one of the pair of cover plates which faces a shaft end surface of a thrust-force acting side of the rotating shaft of one of the gears which receives a thrust force due to a working liquid in the high-pressure and a thrust force due to the meshing from the same direction has a cylinder hole formed at a portion opposite to the shaft end surface thereof, a flow path for supplying the working liquid in the high-pressure side into the cylinder hole is formed, a piston is inserted through the cylinder hole to be capable of being brought into contact with the shaft end surface opposite to the cylinder hole, and the working liquid in the high-pressure side is caused to act on a back surface of the piston to press the piston onto the shaft end surface, thereby causing a drag approximately balancing a resultant force of the two thrust forces to act on the shaft end surface, and on the other hand the one of the pair of cover plates does not have a cylinder hole formed at a portion opposite to a shaft end surface of the rotating shaft of the other of the pair of gears, and the pair of helical gears (20, 23) have a tooth profile fulfilling a condition that a ratio of contact ratios ε r (= ε β / ε α) which is a ratio of overlap ratio ε β to transverse contact ratio ε α is 2 <= ε r <= 3. a pair of helical gears (20, 23) which each have a rotating shaft (21,24) provided to extend outward from both end surfaces thereof, and whose tooth portions mesh with each other, the pair of helical gears having a tooth profile including an arc portion at a tooth tip and a tooth root, and forming a continuous line of contact from one end portion to the other end portion in a face width direction at a meshing portion; a body open at both ends and having a hydraulic chamber therein in which the pair of gears (20, 23) are contained in a state of meshing with each other, the hydraulic chamber having an arc-shaped inner peripheral surface with which outer surfaces of the tooth tips of the gears are in sliding contact; a pair of bearing members (40, 44) which are respectively disposed on both sides of the gears in the hydraulic chamber of the body and which support the rotating shafts (21,24) of the gears so that the rotating shafts are rotatable; a pair of cover plates (7, 8) which are respectively liquid-tightly fixed to both end surfaces of the body to seal the hydraulic chamber, the hydraulic chamber having a low-pressure side defined on one side of the meshing portion of the pair of gears and a high-pressure side defined at the other side thereof; and the body having a flow path which opens into an inner surface of the low pressure side of the hydraulic chamber and a flow path which opens into the inner surface of the high pressure side of the hydraulic chamber, characterised in that one of the pair of cover plates which faces a shaft end surface of a thrust-force acting side of the rotating shaft of one of the gears which receives a thrust force due to a working liquid in the high-pressure and a thrust force due to the meshing from the same direction has a cylinder hole formed at a portion opposite to the shaft end surface thereof, a flow path for supplying the working liquid in the high-pressure side into the cylinder hole is formed, a piston is inserted through the cylinder hole to be capable of being brought into contact with the shaft end surface opposite to the cylinder hole, and the working liquid in the high-pressure side is caused to act on a back surface of the piston to press the piston onto the shaft end surface, thereby causing a drag approximately balancing a resultant force of the two thrust forces to act on the shaft end surface, and on the other hand the one of the pair of cover plates does not have a cylinder hole formed at a portion opposite to a shaft end surface of the rotating shaft of the other of the pair of gears, and the pair of helical gears (20, 23) have a tooth profile fulfilling a condition that a ratio of contact ratios ε r (= ε β / ε α) which is a ratio of overlap ratio ε β to transverse contact ratio ε α is 2 <= ε r <= 3.