To minimize the bending moment of a large float container berth having cranes movably placed thereon, operating on floats with the movement of the cranes, and to secure the structural strength thereof.

The amount of water in ballast tanks is adjusted depending on the positions of the cranes to reduce the bending moment generated in the floats. Furthermore, water feeding means for putting water in or out of the ballast tanks or externals of the floats is installed so that the ballast tank to be subjected to water decrease or increase is specified in accordance with positional signals for the cranes and water amount signals for the ballast tanks, the amounts of water decrease and increase in the ballast tanks are computed, and the amount of water is controlled in accordance with the computation result.