PURPOSE: To facilitate the constitution of an open loop hydraulic circuit by forming an oil chamber in the outside of a pump housing, and interconnecting this oil chamber to each port via a check valve and a relief value, in a device which reverses an inlet port and a discharge port and vice versa according to a direction of rotation.

CONSTITUTION: In a trochoid pump that housed an external gear 1 free of translational rotation in an internal gear 3, a case 21 formed with an oil chamber inside is solidly attached to a pump housing 4 with an inlet port 6 and an discharge port 7, and check values 11, 12 and relief valves 13, 14 are installed between the oil chamber 20 and these ports 6, 7. In addition, a drain 19 leading a hydraulic fluid to be exposed beyond a bearing 9 from the expanding oil chamber in the pump housing 4 is formed so as to be opened to the oil chamber 20 after piercing through the pump housing 4, while a port 22 connecting the oil chamber 20 to an unillustrated tank is formed in the case 21.