Provided is a method of producing an inoculum, which can drastically reduce a time to complete a reaction for mineralizing an organic material into nitrate nitrogen and can add a large amount of the organic material at one time in a multiple parallel mineralization for generating nitrate nitrogen as inorganic nutrients from the organic material, resulting in efficient generation of a high concentration of nitrate nitrogen and drastic reduction of the amount of a microorganism source added. The method of producing an inoculum comprises: placing water in a container that can store water therein, inoculating microorganisms capable of conducting a multiple parallel mineralization thereinto, and maintaining an environment that allows the multiple parallel mineralization to proceed in the water, thereby culturing the microorganisms capable of conducting a multiple parallel mineralization; forming a biofilm on a solid surface that contacts with the water and then collecting the biofilm; and utilizing the collected biofilm as an inoculum of the microorganisms optimized as a catalyst for the multiple parallel mineralization.