To decrease BOD in organic waste water and to generate hydrogen and methane.

The organic waste water is introduced via a conduit 5 into a bioreactor 1 filled with facultative anaerobes which are a kind of hydrogen-producing bacteria. Therein hydrogen is generated by the effect of the hydrogen-producing bacteria and an organic acid is formed which is a substrate of methane. The liquid containing the organic acid is introduced via a conduit 9 into a bioreactor 3 filled with methane bacteria by which the organic acid is decomposed to generate the methane and to decrease BOD. The liquid is then discharged via a conduit 15.