To provide a method of obtaining a standard powder sample of in-water fulvic acid, including less impurities or non-humic materials, in a high yield and in a short period of time.

Natural water is continuously passed through a cartridge filter. Its filtrate is acidified and then run into a resin column to cause it to adsorb an in-water humic material. A resin having adsorbed the humic material is taken out of the column and desorbed by eluting the humic material by a batch method. An eluate is adjusted in pH, then deposition of humic acid is removed by a centrifuge separation method, and a supernatant liquid is run into the resin column to cause the resin to adsorb the fulvic acid. The fulvic acid adsorbed by the resin is desorbed by eluting it by a solution. Its eluate is passed through a positive-ion exchange column and then freeze-dried to obtain fulvic acid powder. The acid powder is sequentially Soxhlet-extracted by a solvent to extract/remove the non-humic materials. After the extraction, the remainder is dried to obtain the standard powder specimen of the in-water fulvic acid.