PURPOSE: To improve the cooling effect of the working oil and reduce a capacity of a tank by a method wherein the oil reservoir is provided along the inner circumference of the outer shell of the underwater cruising body utilized for ocean investigation or the like and the reservoir is partitioned in to a plurality of oil chambers by transversal bulkheads to pass the working oil through the oil chambers sequentially.

CONSTITUTION: The oil reservoir 1 is provided along the inner circumference of the outer shell 11 of the underwater cruising body and this reservoir is constituted into an annular configuration with one part of the outer shell 11 contacting with the external water and an inner shell 1a located at inner side of the outer shell. A plurality of the annular oil chambers 1b partitioned by the transversal bulkheads 16 are formed in the reservoir 1. The internal space of each oil chamber 1b is partitioned further and partitioning members 1c having returning passageways 18 and communicating holes opened at the transversal bulkheads 16 adjacent to these members 1c are provided therein. The working oil is circulated by a pump 3 into the circumferential direction of each oil chamber 1b and is flowed into an arrow sign direction through the returning passageways 18 while passing through each oil chamber 1b sequentially.