An object of the present invention is to produce a pharmaceutical preparation containing, as an active ingredient, a demethyl polymethoxyflavonoid having an excellent physiological activity by a method, for which a raw material is readily available and which includes a simple step and is highly safe for oral ingestion. Further, provided is a production method for 4'-demethylnobiletin or 4'-demethyltangeretin, including fermenting a skin or its water extract product using one or more Aspergillus molds selected from Aspergillus kawachii, Aspergillus awamori, Aspergillus oryzae, Aspergillus sojae, Aspergillus saitoi, and Aspergillus usamii to obtain a fermented product, wherein the skin is derived from at least one citrus fruit selected from citrus fruits belonging to section Acrumen in subgenus Metacitrus in genus Citrus or citrus fruits belonging to section Aurantium in subgenus Archicitrus in genus Citrus.
