TECHNOLOGY FOCUS - BIOTECHNOLOGY - Preferred Device: The device further comprises particle measurement device capable of measuring particle, which, when the number concentration of parasite in raw water is the number concentration higher than the number concentration constant setting value of raw water of the water purification field, continuously counts the particles of the size more than detected object parasite size in filtered water in water purification treatment filtering procedure with the particle measurement device, and controls the operation condition of water purification procedure to maintain water quality of water purification treatment based on measurement result. The preset number concentration setting value can be set in any way according to the removing ability of water purification device. The parasite is Cryptosporidium hominis. The parasite number concentration measurement system is set with separation and concentration part comprising: (1) a filter; (2) antibody reaction part supplying labeled antibody specifically bound to parasite in a sample so that parasite is bound with labeled antibody through reaction, where the sample is obtained through recycling the particles containing parasite and captured on the filter surface; and (3) detection part for detecting parasite bound with labeled antibody. Preferred Method: In the method for monitoring water purification treatment, the measurement of parasite number in the raw water comprises: (1) filtering raw water in separation and concentration part; (2) recycling the particles containing parasite and captured by the filtering of separation and concentration part; and (3) enabling antigen and antibody to react through fluorescence labeled antibody specifically bound with detected object parasite; and (4) judging the existence of parasite through fluorescence detection. When the number concentration of parasite in raw water is the number concentration higher than the number concentration constant setting value of raw water of the water purification field, the method comprises counting the particles of the size more than detected object parasite size in filtered water in water purification treatment filtering procedure through the measurement of parasite in the raw water, and controlling the operation condition of water purification procedure to maintain water quality of water purification treatment based on measurement result. The method comprises: (1) taking raw water and agglutinating in mixing pool; and (2) separating turbidity component in deposition pool after the forming of flocculate to obtain filtered water with filtering procedure. The operation condition of the water purification procedure comprises: (1) the operation of controlling water taking quantity; (2) the operation of stopping water taking; (3) the control operation of enabling the injection rate of agglutinant to increase; (4) the control operation of enabling the stirring strength G value of flocculate forming pool to increase; (5) the operation of exhausting the exhaust water into sewer during inverse cleaning of filtering procedure; (6) the operation of inactivating the parasite concentrated by filtering procedure; or (7) the operation of removing parasite through membrane filtration and reflowing to raw water.