$SiO_2$-ionized loess was prepared from the reaction of loess and sodium hydroxide at $1,400^{circ}C$. The antifungal activity of $SiO_2$-ionized loess against Trichophyton tonsurans, causing tinea, was examined by comparing against that of untreated loess. Soap containing $SiO_2$-ionized loess was used for clinical efficacy tests in the evaluation of the treatment of tinea pedis. The mineral components in loess were maintained after the reaction and the content of ignite loss was decreased to 0.64%, compared to loess. The antifungal ratio reached 56.8% on medium containing 2.0% $SiO_2$-ionized loess and 81.1% on medium containing greater than 3.0% $SiO_2$-ionized loess. However, medium containing untreated loess had no antifungal activity. A clinical efficacy test revealed that soap containing the $SiO_2$-ionized loess led to a significant improvement in symptoms, with the efficacy increasing as the duration of treatment increased.