PURPOSE: To store energy in the form of pressurized air, by making the container of the pressurized air from a flexible watertight film material wherein the container is connected to an air compressor.

CONSTITUTION: An air feeding pipe 3 is connected to an air compressor 2, and other end of the air feeding pipe 3 is connected to the container 1 of pressurized air. The container 1 is made from a flexible watertight film material, so that the air volume in the water is made variable. The buoyancy of the container 1 and a weight 11 are balanced so as to hold the container 1 in the water. On the generation of electrical energy, the pressurized air is taken out to obtain driving force. Thus, energy can be stored in the form of pressurized air until the generation of electrical energy.