PURPOSE: To permit fermentation of raw materials contg. water at high rates by charging only the sewer sludge, etc. into a fermentation tank charged with additives under supply of air, and accelerating their fermentation by mixing then continuing the mixing and the air supply until the mixture matures fully.

CONSTITUTION: Sewer sludge, screening sludge etc. are charged into a fermentation tank which has prescribed volume and is provided internally with a mixing mechanism, and predetermined additives are charged therein. Org. materials of low moisture contents such as sawdust, chaff, etc. other than the reflux materials subjected to the fermentation treatment are used for the additives 2. After only the additives 2 are charged at a predetermined amt., sewer sludge, etc. 1 are charged into the fermentation tank. Even when the moisture contents into the fermentation tank are as high as ≤70%, the sewer sludge, etc. are mixed with the additives simultaneously, then the moisture contents decrease down to about some 40%, and at around the end of a day, the bulk of the total of the sewer sludge and the additives decreases by some 10% than that when they are charged.