PURPOSE: To evenly lower the temperature of work oil by providing a spiral plate between an inner cylinder having a hydraulic pump therein and a cylindrical oil tank, and a return port on the bottom of a double cylinder.

CONSTITUTION: An inner cylinder 2 having a hydraulic pump 5 therein is concentrically provided in a cylindrical oil tank 1, while a spiral plate 3 is provided therebetween, and, a discharge port 11 is arranged on a bottom plate 6. A circulating flow passage is therefore formed in a fully closed structure and increased in its length. The stay time of work oil is then extended and outside pressure is reduced through suction of the hydraulic pump 5, thereby separation of bubbles in the work oil is expedited and generation of cavitation can be prevented. Further, temperature of the circulated work oil is evenly lowered since radiation time to the atmosphere is increased.