PURPOSE: To discriminate mud and sand effectively from sea water, and to prevent the mud and sand from being incorporated into a ballast tank by installing a cyclon separator in the suction side of a ballast pump, and by forming a large suction diameter from the separator to a suction tube on the intake side.

CONSTITUTION: When sea water is taken in from a sea chest 2 by driving a sea water pump 10, sea water including dust or mud and sand is taken in from a sea water suction tube 3 at a flow velocity of approximately 3m/sec into a cyclon separator 5, and is formed into rotating current in the separator, and since centrifugal force is given thereto, mud and sand of larger specific gravity are precipitated downward, while sea water including neither mud nor sand is guided from the upper part of the center of the separator 5 to a suction tube 7. The sea water is guided into a suction tube 9 of small diameter, through a taper tube 8, the suction diameter of which is larger in the intake side, and is accelerated, while the sea water is energized by the sea water pump 10, and is fed into a ballast tank through a feeding tube 11. The mud and sand precipitated in the bottom of the separator 5, are drawn out by appropriately opening a gate valve 6.