Abstract: A method of manufacturing a column reactor for producing nitrate nitrogen as an inorganic nutrient from an organic material including: filling a container with a carrier comprising rockwool, vermiculite, pearlite, zeolite, sand, glass, ceramic, urethane, nylon, melamine resin, cedar chips, bog moss, filter paper or agar; adding thereto microorganisms capable of conducting a multiple parallel mineralization by mineralization of an organic material to produce nitrate nitrogen; subsequently adding 0.01 to 20 g in terms of dry weight of an organic material with respect to 1 L of the carrier, the organic material comprising fish-based soluble fertilizer, fish flour, oil cake, raw garbage, corn steep liquor, rice bran, soybean flour, plant residue, milk, powdered milk or livestock manure; and leaving the resultant material at rest until nitrate nitrogen starts to be produced in an effluent during a washing of the carrier by addition of water to discharge the effluent from the carrier.