Abstract: Provide is a medicinal agent having a strain-selective antibacterial activity for acne bacteria which are indigenous bacteria in the human skin, namely having a stronger bactericidal or growth suppressing activity on an acne bacterium strain enriched in an acne patient group rather than on an acne bacterium strain enriched in a healthy subject group. An acne strain-selective antibacterial agent, comprising at least one fatty acid selected from the group consisting of saturated fatty acids having 14 to 20 carbon atoms (C14:0 to C20:0) and a monounsaturated fatty acid having 18 carbon atoms (C18:1) in a free form or a salt form, or an ester thereof.