When marine organisms become entrapped in the ballast water of ships, they are carried to regions of the sea that are different from where they originated. Once released, those marine organisms may destroy the ecosystem in the area to which they are transferred, with adverse consequences likely. This represents a new maritime pollution problem. A strategy to control this problem is under examination at the IMO (International Maritime Organization). In addition, in numerous countries there is a growing movement to enact municipal laws requiring the management of ballast water in ships entering a domestic sea area. Such laws require ballast water to be exchanged in the ocean a certain distance from land. However, ballast exchange in the ocean can, under certain circumstances, endanger ship hulls. There are several candidate operations to exchange ballast water at sea : the Sequential method, by which all of the ballast water is pumped out from tanks one by one; the Flow-through method, by which the ballast water is pumped up from the tank to overflow across the deck and is replaced gradually by new sea water; and so on. The Sequential method has the advantage of speed over the Flow-through method. However, some issues with the Sequential ballast exchange method remain to be resolved. One of these problems is the sloshing of ballast water in the tank. In the Sequential method, the volume of ballast water in the tank varies greatly, from full to empty, during the ballast exchange operation. Therefore, there is a danger of generating resonant motion of water in the tank; intensive sloshing may occur during the ballast-water exchange operation. For this reason, it is necessary to carefully examine whether or not an exchange operation of ballast water in the ocean is possible and which restrictions to the exchange operation may or may not be acceptable. In this paper, the ballast water exchange in irregular seaways is simulated by using a numerical-simulation of sloshing that the authors have developed. The obtained characteristics of the sloshing of ballast water and of the safety of the tank structures are discussed.