NOVELTY - Sealed bioreactor comprises reactor tank, inert gas supply apparatus connected to the reactor tank through openable-closable valve, reaction liquid storage tank connected to the reactor tank and the inert gas supply apparatus through openable-closable valve and that is connected to a first gas recovery apparatus, substrate liquid storage tank that has a substrate liquid supply unit connected to the reactor tank and the inert gas supply apparatus through openable-closable valve, and second gas recovery device connected to the reactor tank.    USE - The sealed bioreactor and method are useful for producing methane and hydrogen gas (all claimed).    ADVANTAGE - The bioreactor can utilize deep-portion underground water of addition zone efficiently.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for the following:    (1) method of producing methane using the bioreactor, involves (a) supplying inert gas to the inside of the reactor tank using the inert gas supply apparatus, and substituting the gas inside the reactor tank with an inert gas, (b) supplying inert gas to the inside of the reaction liquid storage tank using the inert gas supply apparatus, and allowing pressure inside the reaction liquid storage tank higher than the pressure inside the reactor tank, (c) supplying inert gas to the inside of the substrate liquid storage tank using the inert gas supply apparatus, and allowing the pressure inside the substrate liquid storage tank higher than pressure inside the reactor tank, (d) connecting substrate liquid storage tank with the reactor tank, and supplying substrate liquid to the inside of the reactor tank anaerobically from the substrate liquid storage tank using a pressure difference, (e) connecting reaction liquid storage tank with the reactor tank, and supplying reaction liquid to the inside of the reactor tank anaerobically from the reaction liquid storage tank using a pressure difference, and (f) fermenting the substrate contained in the substrate liquid by the microorganisms contained in the reaction liquid, and producing methane inside the reactor tank, where the substrate liquid comprises organic substance, reaction liquid comprises hydrogen generation type fermentation bacteria and a hydrogen utilization methane producer as microorganisms, has salt concentration of 1.5% or less, dissolved oxygen concentration of 0.01%, sulfate ion concentration of 5 mg/l or less, sulfide ion concentration of 0.01 mg/l or less, and iodine ion concentration of 5 mg/l or less, and cell number of the microorganisms is 101-107 cells/ml;    (2) method for producing hydrogen gas using the bioreactor, involves performing the above-mentioned steps (a)-(f); and    (3) private supply system of water/gas/electricity, comprising fuel cell connected to hydrogen gas holder, methane producing portion comprising the bioreactor, hydrogen gas production portion comprising the bioreactor, pump which is connected with the methane producing portion and pumps up deep-portion underground water of an addition zone from deep-ocean-drilling well, methane tank connected to the methane producing portion, hydrogen gas holder connected to hydrogen gas production portion, and generator which has gas engine connected to methane tank.