PURPOSE: To measure a nitration speed more stably, by introducing a part of activated sludge into small aeration tanks and calculating the difference between oxygen consumption speeds in a case injecting a nitration control agent in one of small aeration tanks and a case injecting no nitration control agent in the other one.

CONSTITUTION: A pat of activated sludge from an aeration tank 1 is supplied not only to a nitration control agent non-addition type small aeration tank 3 but also to a nitration control agent addition type small aeration tank 4 in the same amount by a raw water supply apparatus 2. Subsequently, ammonia being the substrate of nitration reaction is supplied to the tanks 3, 4 from a supply apparatus 10a nd the nitration control agent is supplied to the tank 4 from a nitration control agent supply apparatus 11. Further, a constant amount of air is supplied to the tanks 3, 4 through quantitative valves 8 by an air pump 7 and air is supplied to the activated sludges in the tanks 3, 4 by a stirring propeller 6. Aeration exhaust gas is alternately changed over at every definite time by a change-over valve 12 and an oxygen consumption speed is measured by an oxygen analyser 13 and the difference of the oxygen consumption speeds between both tanks 3, 4 is further calculated by an operator 14 to measure a nitration speed.