To solve the problem that in a conventional heat exchanger directly heat-exchanging oil with water via a heat transmission wall when a lubricant or a fuel oil at high temperature is cooled down by water, if sea water is used as cooling water, the heat transmission wall of the heat exchanger is corroded, oil is mixed into sea water, and sea water is contaminated by oil.

A highly reliable heat exchanger has a structure for exchanging heat via a heat pipe, and thus even if a heat transmission wall corrodes owing to sea water, both fluids are not mixed, and sea water is not contaminated by oil.