To provide a positive displacement pimp and/or motor with improved volumetric efficiency.

This positive displacement hydraulic unit 10 has a housing 12 furnished internally with a chamber 30 with circumferential surfaces partitioned by a pair of separated end walls 32 and 34. A rotor 38 to allow a fluid to flow is provided inside the chamber. A covering layer 44 is provided between the end walls of the chamber and the side walls 40 and 42 adjacent to the rotor, and thereby the relief the side face of rotor remains very small, and the volumetric efficiency of the hydraulic unit is enhanced.