NOVELTY - Simulating a biological water treatment comprises: processing the water with the activated sludge in the bioreaction tank comprising (a) following: (i) it is contained in this activated sludge and the number of bacteria for every kind of bacteria; and (ii) activity required from decomposition rate of quality of process target object in this activated sludge; (b) condition component value of water-treatment process; (c) specification of bioreaction tank used as object; (d) running condition of bioreaction tank; and (e) number of bacteria for every kind of these bacteria.    USE - The method is useful for simulating the biological water treatment which processes the waste-water containing a nitrogen compound as to-be-processed water (claimed).    ADVANTAGE - The present invention can be performed at a low cost and it is efficient.    DETAILED DESCRIPTION - Simulating a biological water treatment comprises: processing the to-be-processed water with the activated sludge in the bioreaction tank which has a to-be-processed-water phase containing an activated sludge and to-be-processed water comprising: (a) following (i) and (ii): (i) it is contained in this activated sludge and 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 is determined by a gene analysis method; the bacteria concentration computed from the number of bacteria for every kind of these bacteria; and (ii) activity required from decomposition rate of quality of process target object in this activated sludge, the active parameter computed from these; (b) condition component value of water-treatment process; (c) specification of bioreaction tank used as object; (d) running condition of bioreaction tank; and (e) number of bacteria for every kind of these bacteria, based on these, the quality of a treated water is simulated with an activated-sludge model. An INDEPENDENT CLAIM is the simulation apparatus of a biological water treatment comprises: (a) containing in an activated sludge and a gene-analysis means to 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; a gene-analysis base bacteria concentration calculation means to compute the bacteria concentration based on a gene analysis from the number of bacteria for every kind of bacteria determined by this gene-analysis means; an activity-measurement means to measure activity from the decomposition rate of the quality of a process target object in this activated sludge; an active parameter calculation means to compute an active parameter from this bacteria density and activity of bacteria, the active parameter database with which the data of the active parameter were stored has an activated-sludge model, the data of the active parameter is stored in the active parameter database, the condition component value of the inputted water-treatment process, the specification of a bioreaction tank, and the running condition of a bioreaction tank; calculating a treated water requires the quality of a treated water with this activated-sludge model based on the active parameter obtained by the active parameter calculation means, and the number of bacteria for every kind of bacteria determined by this gene-analysis means.