To provide a ship coping with a problem of ballast water by attenuating the pitching and rolling of the ship.

Tanks with the same capacities are disposed at both boards of the ship. Each one end of an inlet/outlet port of the tank is near a ship bottom to communicate with seawater outside the ship, and the other end thereof communicates with the atmosphere. The ship has the tanks including valves making and breaking the inflow/outflow of air to make the seawater outside the ship flow in and flow out of the tanks.