To reduce influence of buoyancy changed by storage amount of a material to be stored when storage apparatus is installed ion the water.

This underwater storage 3 is composed of a rigid body 10 and a flexible membrane body 11 and is constituted so that closed volume is changeable. A charging and discharging means 2 of the material to be stored is provided with a charged and discharged pipe 20 connected to nearly the center of the rigid body 10, an upper valve 21 provided on the upper end of the charging and discharging pipe 20 and a lower valve 22 provided in the intermediate part of the charging and discharging pipe 20. A buoyancy adjusting apparatus 5 is constituted of a buoyancy adjusting tank 50, an air supply means 51 for supplying air to the buoyancy adjusting tank 50 and a weight 52 attached to the storage tank 3. The flexible membrane body 11 in the tank 3 is moved according to a housed amount of the material to be stored to change the volume in the tank. The storage tank 3 is stably arranged in a prescribed water depth by the density of the material to be stored, air in the buoyancy adjusting tank 50, its own weight of the storage apparatus and the weight 52.