PURPOSE: To recover valuable materials or energy and to contrive reutilization of recovered materials or energy by performing addition and interruption of addition of n-butyric acid in the first stage and adjusting the proportion of n- butyric acid in the formed volatile fatty acid to ≥30%.

CONSTITUTION: Raw material slurry for fermentation is charged to a liquefaction fermentation tank 2 and org. materials are brought into contact with liquefying fermentation microorganism to decompose to volatile fatty acid such as acetic acid or butyric acid, etc. Gaseous H2, gaseous CO2, or gaseous methane are generated simultaneously with the decomposition of org. materials, and generated gas is stored in a gas holder 4. The compsn. of volatile fatty acid generated in the liquefaction fermentation tank 2 is analyzed, and an amt. of n-butyric acid specified depending on the concn. of the compsn. and concn. of the product is charged. n-butyric acid is charged to the liquefaction fertilization tank from an n-butyric acid tank 3 when the proportion of n-butyric acid is below 30% basing on the amt. of whole volatile fatty acids. By this method, generation of gaseous methane in the liquefaction fertilization tank 2 is retarded and generation of H2 is stabilized.