The test equipment started to be operated under the condition of a constant influent water flow rate, the average ambient temperature at 20 ﾟC along with the conditions. After a rotating biological contactor (RBC) was put in operation, the growth of biofilm" starts with adsorbing suspended solids (SS) including heterotrophs and nitrifying bacteria. During operation, microbial communities in the biofilm keep groving and/or decaying and SS in the bulk liquid keep attaching to and detaching from the biofilm surface. After a few week operation, a certain amount of attached biofilm is formed on the substratum. The necessary boundary condition to obtain the rate of the growth of heterotrophs and nitrifying bacteria are oxygen concentration in air and in bulk liquid and substrate (carbon, ammonia, etc) concentration in the bulk liquid. Concentration profiles of heterotrophs and nitrifying bacteria in a biofilm are able to be simulated with these conditions. When the mechanisum of a biofilm formation is understood, the explication and exploitation of the substrate removal process with biofilm formation will be carried out effectively. In this research, simulation on biofilm formation are conduted under several conditions, one of which is that the concentration of active bacteria is low at the biofilm surface increases towrds the substratum. When the rate of bacteria growth and decay, SS attachment and biofilm sloughing get constant, a fixed thickness of a biofilm is obtained. When the concentration of organic matter inbulk liquid is higher, the concentration of active heterotrophs is higher at the surface of the biofilm. When the concentration of organic maiter in bulk liquid is lower, the concentration of active nitrifying bacteria becomes higher at the surface on it. In case of limiting oxygen concentration, aerobes existed up to 100 um in depth of the biofilm.