A method of treating a patient suffering a disease associated with Trichophyton mentagrophytes or Pityrosporum ovale by applying topically an anti-fungal pharmaceutical composition which is prepared from Zingiber officinale, includes the following steps: preparing a crude liquid from rhizomes of ginger by extraction with an organic solvent or supercritical CO₂, or by distillation with steam; introducing the crude liquid to a reverse phase chromatography column, and eluting the column with water, a first eluent and a second eluent having a polarity weaker than that of the first eluent but stronger than that of chloroform, so that a first eluate resulting from elution of the first eluent and a second eluate resulting from elution of the second eluent are obtained; removing the first eluent and the second eluent from the first eluate and the second eluate by evaporation, respectively, so that a first concentrated eluate and a second concentrated eluate are obtained as the potent extract.