The purpose of the present invention is to provide an external preparation which exerts a therapeutic and/or prophylactic effect on infectious diseases in the field of dermatology such as acne accompanied by suppurative inflammation or epidermal skin infectious diseases when administered once a day mainly to a human patient. As an example of the external preparation according to the present invention, an external preparation, which is characterized by comprising as an active ingredient 1-cyclopropyl-8-methyl-7-[5-methyl-6-(methylamino)-3-pyridyl]-4-oxo-1,4-dihydro-3-quinolincarboxylic acid and/or a pharmaceutically acceptable salt thereof and exerting a therapeutic and/or prophylactic effect on infectious diseases in the field of dermatology when administered once a day to a human patient, can be cited.
