PURPOSE: To absorb a change in the volume of operating oil over a wide range of oil temperature by forming an oil chamber, which communicates with an oil storage part, in one part of a casing and providing a piston member, slided by receiving an internal pressure in the oil storage part, in the oil chamber.

CONSTITUTION: Action of a differential limiting part 2 is performed in such a manner that a pressure increase in the interior of each pump chamber 40 and a pressure decrease in a throttle hole 47 are repeated by circulating seal oil in an oil storage part T between the oil storage part T and the pump chamber 40. In the case of an internal pressure increased in the oil storage part T following a rise of oil temperature, a piston member 6 is slided in a direction of separating from a side plate 32 by increasing pressure receiving force, and as a result of increasing the volume of a cavity part 5 in its communicating side with the oil storage part T, the internal pressure is suppressed from rising by absorbing the volumetric expansion of operating oil. Reversely in the case of the internal pressure decreased in the oil storage part T following a decrease of oil temperature, a decrease of the internal pressure is suppressed by absorbing the volumetric contraction of operating oil. By the constitution in the above, miniaturization of the total unit shape can be attained without generating an increase of the axial length dimension.