A phorbol ester component is removed by decomposition at a low cost with a high degree of treatment capability from an organic substance containing a phorbol ester component, whereby a high-protein organic substance is produced.A phorbol ester is decomposed by mixing an organic substance containing a phorbol ester and Bacillus substilis var. natto and subjecting the resulting mixture to fermentation. At this time, 4 parts by mass of organic substance containing a phorbol ester is mixed with 0.5 to 3 parts by mass of water, and the resulting mixture is subjected to high-temperature and high-pressure sterilization. Then, a solution obtained by dissolving 0.004 to 0.2 part by mass of Bacillus subtilis var. natto in 0.5 to 1 part by mass of water is added, and the resulting mixture is subjected to fermentation at 30 to 50° C. for two to four weeks.