A method of producing an inoculum having the following characteristic; a characteristic such that the inoculum maintains its function as the inoculum of the microorganisms optimized as a catalyst for the multiple parallel mineralization when the inoculum is heated at 50 to 80°C, wherein the multiple parallel mineralization is a reaction for successively performing degradation of an organic material into ammonium nitrogen and nitrification of ammonium nitrogen into nitrate nitrogen in the same reaction system as well as a reaction for generating nitrate nitrogen by degrading organic nitrogen contained in the organic material into ammonium nitrogen and then performing nitrification of ammonium nitrogen through nitrification in degradation of the organic material; the method comprising: placing water in a container that can store water therein, adding the following microorganism source containing microorganisms capable of conducting a multiple parallel mineralization; at least one microorganism source selected from the group consisting of soil, compost, activated sludge, and water collected from nature; wherein the microorganisms are selected from microorganisms capable of conducting ammonification and microorganisms capable of conducting nitrification; maintaining an environment in water to satisfy all the following conditions to, thereby culturing the microorganisms capable of conducting a multiple parallel mineralization; a water temperature condition of 15 to 37°C: a condition of maintaining an aerobic condition by aeration, shaking, or aeration and shaking: a condition of adding the following organic material in an amount of 0.01 to 2 g in terms of dry weight per L of the water per 1 to 14 days; at least one organic material selected from the group consisting of fish-based soluble fertilizer, corn steep liquid, oil cake, fish flour, milk, soybean cake, yeast cake, sake cake, shochu cake, and raw garbage; and a condition of stopping addition of the organic material when a concentration of a nitrate ion generated in the water reaches 10 to 50 mg/L; forming a biofilm on the following solid surface that contacts with the water and then collecting the biofilm; at least one solid surface selected from the group consisting of a wall surface of the container and a bottom surface of the container; and utilizing the collected biofilm as an inoculum of the microorganisms optimized as a catalyst for the multiple parallel mineralization. placing water in a container that can store water therein, adding the following microorganism source containing microorganisms capable of conducting a multiple parallel mineralization; at least one microorganism source selected from the group consisting of soil, compost, activated sludge, and water collected from nature; wherein the microorganisms are selected from microorganisms capable of conducting ammonification and microorganisms capable of conducting nitrification; maintaining an environment in water to satisfy all the following conditions to, thereby culturing the microorganisms capable of conducting a multiple parallel mineralization; a water temperature condition of 15 to 37°C: a condition of maintaining an aerobic condition by aeration, shaking, or aeration and shaking: a condition of adding the following organic material in an amount of 0.01 to 2 g in terms of dry weight per L of the water per 1 to 14 days; at least one organic material selected from the group consisting of fish-based soluble fertilizer, corn steep liquid, oil cake, fish flour, milk, soybean cake, yeast cake, sake cake, shochu cake, and raw garbage; and a condition of stopping addition of the organic material when a concentration of a nitrate ion generated in the water reaches 10 to 50 mg/L; a water temperature condition of 15 to 37°C: a condition of maintaining an aerobic condition by aeration, shaking, or aeration and shaking: a condition of adding the following organic material in an amount of 0.01 to 2 g in terms of dry weight per L of the water per 1 to 14 days; at least one organic material selected from the group consisting of fish-based soluble fertilizer, corn steep liquid, oil cake, fish flour, milk, soybean cake, yeast cake, sake cake, shochu cake, and raw garbage; and a condition of stopping addition of the organic material when a concentration of a nitrate ion generated in the water reaches 10 to 50 mg/L; forming a biofilm on the following solid surface that contacts with the water and then collecting the biofilm; at least one solid surface selected from the group consisting of a wall surface of the container and a bottom surface of the container; and utilizing the collected biofilm as an inoculum of the microorganisms optimized as a catalyst for the multiple parallel mineralization.