PURPOSE: To obtain a process measuring device capable of automatically and continuously measuring the activity of nitrification bacteria by forming the device with a means for detecting the oxygen consuming rate by addition of an ammonium salt and allylthiourea and a means for measuring the activity of the nitrification bacteria based on the dissolved oxygen content signal.

CONSTITUTION: Ammonium chloride and allylthiourea are added to activated sludge, and the respective oxygen consuming rates are measured to obtain nitrification activity. Meanwhile, an MLSS meter is provided, and its signal is inputted to calculate and measure the nitrification activity per unit amt. of sludge. Furthermore, a water temp. signal is inputted from the temp. coefficient of nitrification activity and its function to calculate and measure the number of nitrification bacteria per unit amt. of activated sludge. Consequently, the nitrification activity per unit amt. of sludge is automatically and continuously measured.