PURPOSE: To keep the balance of oil pressure acting on an end surface of a rotor, to reduce the abrasion of a connecting part between the rotor and a vibration rod connected to this rotor, and to cool the connecting part in a gerotor-type hydraulic device comprising a housing, the rotor having two flat axial end surfaces, a plurality of gerotor cells, an inlet and an outlet.

CONSTITUTION: A gerotor-type hydraulic device comprises a main housing member 60, an abrasion plate 61, a gerotor set 62, a manifold 63 and a cover 64. The main housing member 60 comprises an inlet 66 continuously connected to an annular chamber 68. The annular chamber 68 is communicated to a radial through hole 69, and this through hole 69 is connected to a hollow part 70a of a driving shaft 70. A central opening 61a of the abrasion plate 61 forms a part of a fluid passageway. Six pairs of fluid passageways 82, 83 penetrating through a rotor 72 connect the central hole 61a to an annular fluid passageway 84. This annular fluid passageway 84 is opened toward the manifold 63.