PURPOSE: To prevent a defective rotational operation caused by a change in the viscosity of sucked and discharged liquid in an inscribed gear pump provided to restrain the increase in the discharge quantity along with the increase in rotational frequency of the gear.

CONSTITUTION: A maximum volume chamber 7 is formed in the opposite side to the deepest gearing position of both gears 5, 6 provided to make inscribed gearing with each other. A suction chamber 8 communicating to a suction port 9 having a throttle 10 is formed on the counter-rotational side of gear from the chamber 7. Contraction chambers 15A, 15B and further a discharge chamber 13 reducing the volume along with the rotation of gears are formed on the rotational side of the gears. Check valves 17A, 17B... communicating to each other are provided on the way of a throttle groove 16 affording a throttle and communication between the contraction chambers 15A... and a discharge port 14. When the outflow of liquid in the throttle groove 16 is reduced by a change in the viscosity of the liquid, the check valves 17A... are opened to force the liquid to short circuit and flow out so that pressure in the contraction chambers 15A... is prevented from excessive increase.