To form a mechanism capable of two-way operation which make high-pressure side chamber and low-pressure side chamber switchable by two tooth top seals.

In a pump housing 1, gears 8, 9 are rotatably supported by side plates 5, 6 and pressure chambers 15, 16 are formed by tooth top seals 12, 14 at both sides of meshing parts S. During pump operation, the pressure chambers 15 and 16, become low/high pressure chamber and high/low pressure chamber corresponding to the suction/delivery side and delivery/suction side caused by rotating direction of gears 8, and 9, so that oil is transferred from high pressure chamber to low pressure chamber through intermediate chamber 17. Thus, the gear pump can switch the suction and delivery sides to each other depending on rotating direction of gears 8, 9, and capable of forming two-way operation with simple structure.