in an anaerobic treatment method in which contaminated water is introduced into an acidification tank to decompose polymer components in the contaminated water into organic acids , and the effluent from the acidification tank is introduced into a methane production tank containing non - biological fluidized carriers so as to convert the acids into methane , the treatment is carried out stably and continuously by preventing the occurrence of floating of carriers in the methane production tank . the cod cr concentration of polymer components other than organic acids in the acidification tank effluent to be introduced into the methane production tank is controlled to be not more than 300 mg / l . this control prevents the occurrence of acid production reactions in the methane production tank and the consequent proliferation of microorganisms , and thereby suppresses sticking of carriers as well as the occurrence of floating and blockage by the carriers .