TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Method: The pH of the slurry prior to wet oxidation process is adjusted, so that the pH of the slurry after the wet oxidation process is 6.5-7.5. The processed material which is passed through the wet-oxidation process, separates the oxidation slurry into at least one portion of the oxidation supernatant liquid which is a liquid phase. The microorganism decomposition process is an anaerobic digestion process which performs an anaerobic digesting, or hydrogen fermentation process using anaerobic hydrogen producing microbe. EXAMPLE - Newspaper (20 g) containing cellulose (in mass %) (54) and lignin (16.9), was cut and slurried after adding distilled water (1 l). Sodium carbonate (10 g/l) was added to the newspaper slurry, so that the pH after wet oxidation was 6.5-7.5. The wet oxidation treatment of the newspaper slurry was performed on alkaline conditions, in an shaking-type autoclave test equipment at 210 degrees C 1 hour with oxygen supply amount per newspaper slurry unit volume which corresponds to 20% of chemical oxygen demand by potassium dichromate (CODcr value) of the slurry. The pH of the slurry was 7.17. Alternatively, the wet oxidation treatment of the newspaper slurry was performed on acidic conditions with acid (20 g/l) at 210 degrees C for 1 hour, in an shaking-type autoclave test equipment with oxygen supply amount per newspaper slurry unit volume, by sodium carbonate additive free. The processed material was obtained. The decomposition efficiency of lignin was high (99%) at 210 degrees C in alkaline conditions then acidic condition. The decomposition efficiency of cellulose was 71% at 210 degrees C in alkaline condition. Generation of furan such a hydroxymethyl furfural which exerts an inhibitory effect with respect to microorganisms, was sharply reduced on alkaline conditions compared with alkaline condition. Anaerobic digestive processing with more stable wet oxidation treatments was performed in alkaline conditions.