TECHNOLOGY FOCUS - MECHANICAL ENGINEERING - Preferred Apparatus: H2 and O2 gas storage apparatus (39, 57) consists of several H2 and O2 gas containers (41-1-41-n, 59-1-59-n) of small capacities connected in parallel through separate switch valve. The switch valve has remote control which can be operated individually. A pressure sensor for detecting container gas pressure is provided in each H2 gas container. The O2 gas deriving system line is provided with O2 gas deriving switch valve (55). H2 and O2 gas supply switch valves (49, 67) open and close the supply of H2 and O2 gas from the respective apparatus. At least one deriving system line is intervened between H2 and O2 gas deriving system lines. H2 and O2 gas produced by disassembly of water in the electrolytic cell is stored in the respective gas storage apparatus. The equal pressure device (69) is connected between the O2 and H2 gas deriving switches and is divided into H2 gas chamber (71) and the O2 gas chamber (73) supplied with the corresponding gases from the respective deriving system lines. The H2 and O2 gas chambers so composed such that the volume becomes relatively variable by the pressure differential of each gas. Displacement detection units (77) are provided in device (69) for measuring the difference of H2 and O2 gas pressures. The detection units (77) detects an expansion or bending of a partition material (75) or structure and the maximum threshold position of bending. The amount of displacement of the site is detected by the site which displaces in connection with relative change of the volume of each chamber in an equal pressure device. H2 and O2 tanks (7,15) for vapor-liquid separation and water level detection units (19) for detecting water level in H2 tank and O2 tank are provided in the apparatus. Based on water level detection results, at least one water level of both tanks is adjusted by water level adjustment units so that the volume of gas phase on water surface of H2 tank is doubled with volume of gas phase on water surface of O2 tank. The H2, O2 and high pressure-vessel are composed of FRP. The apparatus consists of an installation which supplies electric power for performing electrolysis of water cell and it has at least one installation among electric power receiving installations such as solar power generation and wind power installations. The apparatus consists of a fuel cell. Preferred Composition: Each H2 and O2 gas containers are composed of fiber reinforced plastics (FRP) formed by reinforcing an aluminum and/or alloy by gas fiber or carbon fiber. Preferred Condition: Pressure in the water electrolytic cell and in high-pressure vessel are equal. The volume of H2 gas stored in apparatus (39) is double the volume of O2 gas stored in O2 gas storage apparatus. The volume of each gas chamber changes in the direction where the differential pressure of gas of H2 and O2 gas chambers is 0. Preferred Arrangement: The apparatus is installed on roof of a gasoline station building. Preferred Material: A flexible pipe (18) having a elasticity or flexible material or structure capable of expanding and reducing by internal and external pressure differential is provided at the site where the system path outer surface contacts with electric insulation liquid (5) in high pressure vessel.