PURPOSE: To control quickly, adequately and automatically the dissolved oxygen in an aeration tank by attaching DO concn. meters, air flow meters and sludge flow meters to the aeration tank of a water treating device and calculating the required air flow rate adapted to the changing inflow rate of sludge.

CONSTITUTION: A sludge flow meter 12 is provided in the inlet of an aeration tank 1 in a water treating installation by activated sludge and DO concn. meters 8, 2 for measuring the concn. of dissolved oxygen in the aeration tank are attached in the middle of the tank 1 and near the outlet thereof. Air flow meters 9, 10, 11 and control devices 5, 6, 7 for the flow rate of aeration air which control said meters are also provided. A calculator 13 for the DO concn. distribution which calculates the concn. distribution of the dissolved oxygen over the entire part of the tank 1 by the meters 2, 8, the meters 9, 10, 11 and the meter 12 as well as a calculator 3 for the flow rate of the aeration air and a setter 4 for the flow rate of the aeration air are provided. The concn. distribution of the dissolved oxygen over the entire part of the tank 1 can be calculated with the calculator 13 by the above-mentioned device and therefore the amt. of the dissolved oxygen is controlled quickly at the set value even if the inflow rate of the sludge into the tank 1 fluctuates for a short time and the treated water always having stable water quality is obtd.