To provide a fermentation tank which supplies a proper amount of oxygen with a convenient means, proliferates microorganism, and concurrently suppresses the generation of ammonia smell while conveniently removing the generated ammonia smell even with the supply of oxygen.

A fermentation tank is hermetically configured by including an oxygen enricher 5 that accommodates treated matters w1, w2 containing an organic substance which suppresses the generation of ammonia smell that may be generated when a biological decomposition occurs as an amount of oxygen in an ambient atmosphere increases in the inside and separates oxygen from the atmospheric air and supplies the same into the fermentation tank, and an oxidation and denitrification means 6 that oxidizes the ammonia into nitrogen and converts nitrogen oxide partially produced by the oxidization of the ammonia into nitrogen. In addition, electrodes 51, 52 are formed on both surfaces of a sheet-like solid electrolyte as the oxygen enricher having a power supply that supplies direct current to the electrodes.

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