Provided is a skin external preparation including a drug and nanometer-size molecular assemblies. The molecular assemblies contain an amphiphilic block copolymer having a hydrophilic block chain including a sarcosine-derived structural unit and a hydrophobic block chain including a hydroxy acid-derived structural unit. The skin external preparation according to the present invention may reduce skin irritation caused by the drug, be very safe, and have excellent pharmacological effects, even when a drug that is generally difficult to be transdermally administered due to its strong skin irritation is contained therein.