PURPOSE: To prevent deterioration of efficiency produced by oil leak in such a way that seal block is followed in response to deflection of gears at the time of high loading, and proper addendum clearance is maintained.

CONSTITUTION: Addendum tangential face side of a seal block 31 which comparts a high pressure chamber 36 is formed as a flexible part 41, the high pressure chamber 36 is set to be able to respond to wider range than in the tooth groove high pressure range of gears 19, 20. When it comes to be under high loading, the gears 19, 20 deflect in the direction estranged also from the seal block 31, but because it leads to high pressure in the high pressure chamber 36, also the flexible part 41 expands in the directions of the gears 19, 20, the addendum clearance is decreased, and oil leak is reduced, so that the gear pump is prevented from the deterioration of efficiency.