[ $SiO_2$ ]ionized loess was prepared from the reaction of loess and sodium hydroxide at $1,400^{circ}C$ for 2 h. The antibacterial activity of $SiO_2$-ionized loess against Staphylococcus aureus, Bacillus subtilis, Escherichia coli, Pseudomonas aerusinosa, and Propionobacterium acnes causing acne was examined by comparing the results against those of untreated loess, and treatment efficacy was examined for treating acne using soap containing $SiO_2$-ionized loess. The minimum inhibitory concentration value of $SiO_2$-ionized loess against S. aureus, P. aerusinosa, B. subtilis, E. coli, and P. acne was 10.0, 10.0, 2.5, 5.0, and 2.5 mg/mL, respectively. However, medium containing untreated loess had no antimicrobial activity. A treatment efficacy test revealed that acne symptoms decreased as the duration of using soap containing $SiO_2$-ionized loess increased.