To mitigate inconvenience such as pressure loss or increase due to deposition of salts in an air cooler.

A seawater supply pipe 40 and a seawater exhaust pipe 45 are attached to an air cooler 20, and the seawater transferred from a pump 48 is introduced to the air cooler 20 through the seawater supply pipe 40. The seawater whose temperature is raised by cooling combustion air is exhausted from the seawater exhaust pipe 45 to the outside of the ship. A flow rate adjusting means, namely a rotary valve 50 is attached the seawater supply pipe 40 and the seawater exhaust pipe 45. In response to the feedback of the temperature of combustion air after cooling measured by a temperature meter 90, an opening degree of the rotary valve 50 is adjusted by a control device 200.