PURPOSE: To prevent component members from being corrected by sea water by constituting a sea water pump so that the sea water in the pump component members of the sea water pump can be discharged by the atmosphere via a gas connecting line connected upstream a discharge valve openably provided on the pump discharge side.

CONSTITUTION: A sluice valve 27 of a gas feed line 25 connected to the upstream side of a discharge valve 23 is opened to feed air or N2 gas into a discharge elbow 16 via a check valve 28. When air is injected into an enclosed system in the discharge elbow 16, the sea water in the enclosed system is gradually discharged by the injected atmosphere and is discharged to the outside of a sea water pump 10 through a pump intake port 18. Thereby, the sea water level in the enclosed system is gradually lowered, and finally the discharge elbow 16, a lifting pipe 15 and a pump casing 12 are filled with the atmosphere of air or the like, and sea water is completely discharged. Accordingly, the inside of the pump component members, particularly around an impeller, can be filled with the atmosphere hard to be corroded, thus corrosion of the sea water pump 10 by sea water can be reliably and effectively prevented.