To reduce the size of a fermenter, to shorten the fermentation period and to reduce the treatment cost by lowering the moisture content of a sludge cake to be treated down to a moisture content suitable for fermentation and composting the sludge cake, thereby decreasing the amount of the dry sludge to be fed into the fermenter without using a moisture content regulating material, sawdust, etc.

The sludge cake to be treated is forcibly dried by a sludge drying machine 5 to obtain a dry sludge of moisture content 50 to 60% and is then transferred into a fermenter 11 where the sludge is fermented while the sludge is rotated and overturned. At the point of the time when the sludge is composted upon ending of the fermentation of the dry sludge, the compost is loaded to a container 12.