PURPOSE: To simplify structure, heighten sealing performance and durability and aim at large capacity, that is, compactness in a fluid pressure pump/motor provided with a rotary rotor.

CONSTITUTION: A rotor accommodating chamber 5 of circular cross section is formed in a housing 1, and a rotor 2 is accommodated in this rotor accommodating chamber 5 in such a way as to be rotatable integrally with a shaft member 3. A pressure receiving protruding part 21 with a seal part 22 brought into sealable sliding contact with the inner peripheral surface of the rotor accommodating chamber 5 is integrally formed at the rotor 2, and an operating chamber 50 is formed outside of the rotor 2 in the rotor accommodating chamber 5. The housing 1 is provided with vane mechanism 4 for energizing a movable partition member 41 for partitioning the operating chamber, by a spring 43. The operating chamber 50 is partitioned into a first operating chamber 51 and a second operating chamber 52 by the pressure receiving protruding part 21 of the rotor 2 and the movable partition member 41, and a first port 6a and a second port 7a positioned near both sides of the vane mechanism 4 are formed at the housing 1.