PURPOSE: To prevent deterioration of activity of sludge in a time of low load and to stably perform treatment in the case of treating organic substance by anaerobic bacteria showing activity at a high temperature region.

CONSTITUTION: A sludge storage tank 7 for storing sludge at low temperature is connected with an anaerobic treating tank 2 at high temperature so that sludge is transferred between the treating tank and this storage tank. Thereby load close to maximum load is imparted to sludge in the treating tank 2 to which load is imparted and activity is maintained by performing a suitable control even under the low load condition and also that residual sludge is held at low temperature at the storage tank 7 and thereby activity is maintained. Therefor even raw water large in load fluctuation is stably and efficiently treated.