PURPOSE: To effectively perform the titled operation by a system wherein one tank of multi-ballast tanks having a double bottom is enclosing a submerged pump, a sea chest formed in the ship's bottom and respective ballast tank are connected by change-over valve and piping.

CONSTITUTION: A submerged pump 43 is provided for the bottom of one ballast tank 39 of the tanker where multi-divided double bottom tanks 35, 37, 39, 41 are used as the exclusive ballast tanks. Respective ballast tank and the sea chest 51 located at th ship's bottom are connected by change-over valve and piping. On filling water, the sea water sucked from the sea chest 51 is delivered to respective ballast tank by passing through the valves 65, 57, pump 43, valve 67, 59 and piping 53, 55. At discharging, they are discharged to the ouside of the ship by passing through piping 55, 53, valve 61, pump 43, valves 63, 65 and sea chest 51.