To prevent the pollution of soil or seawater caused by treating ballast water by supplying ballast water for keeping the stability of a ship to the surface of the sea to a fixed bed type electrode electrolytic cell to treatmicroorganisms in ballast water.

A ballast tank 301 for housing ballast water 303 is disposed in the ship body 300 of a ship. The ballast water 303 is introduced into or discharged from the ballast tank 301 by the action of a pump 306 through a piping 320 and treated in the fixed bed type electrode electrolytic tank 304 disposed on the way of the piping 320 to kill microorganisms in the ballast water. The treated ballast water is supplied to the ballast tank 301. When the ship is loaded with ballast water 303 to navigate and arrives at a destination, a cargo is housed in the cargo storehouse 302 of the ship and, at this time, the ballast water 303 is discharged out of the ship through the piping 32 by the action of the pump 306. At this time, microorganisms in the ballast water 303 are killed by the fixed bed type electrode electrolytic cell 304 disposed in the route of the piping 320.