TECHNOLOGY FOCUS - BIOTECHNOLOGY - Preferred Method: The simulation method of the biological water treatment comprises (a) determine the number of bacteria for every kind of bacteria which are concerned in decomposition of the quality of a process target object in this to-be-processed water by a gene-analysis method contained in the activated sludge; (b) computing bacteria concentration based on gene analysis from number of bacteria for every kind of bacteria determined at the simulation; (c) measuring activity from decomposition rate of quality of process target object in the activated sludge; (d) computing an active parameter from activity computed by (c) and the bacteria concentration computed at (b); and (e) simulating the quality of a treated water with an activated-sludge model based on the number of bacteria for every kind of bacteria determined at (a), active parameter computed at (d), condition component value of water-treatment process, specification of bioreaction tank used as object, and running condition of bioreaction tank. The activated-sludge model is improved based on IWA activated-sludge model NO.3 (ASM3) and the gene-analysis method is real-time PCR. The activity which is concerned in decomposition of the quality of a process target object corresponding to these bacteria is restoration activity which is concerned in decomposition of oxidation activity and quality of a process target object, where the bacteria exhibit an oxidation capacity, this quality of a process target object, and/or exhibit reduction ability. It may measure the denitrifying activity containing nitrate-reduction activity and nitrous acid reduction activity as this reduction activity. The to-be-processed water is a waste-water containing a nitrogen compound. Preferred Apparatus: The activated-sludge model is improved based on ASM3. It is a means by which the gene-analysis means determines the number of bacteria for every kind of these bacteria by real-time PCR. It may also measure reduction activity which is concerned in decomposition of oxidation activity which is concerned in decomposition of this quality of a process target object, and/or this quality of a process target object as activity which is concerned in decomposition of this quality of a process target object. The denitrifying activity in which reduction activity is concerned in decomposition of this quality of a process target object contains nitrate-reduction activity and nitrous acid reduction activity and the oxidation activity which is concerned in decomposition of this quality of a process target object is nitrification activity containing ammonia oxidation activity and nitrous acid oxidation activity. EXAMPLE - No suitable example given.