Disclosed is an external preparation composition having good transdermal absorbability.An external preparation composition having excellent transdermal absorbabilitycan be produced by dissolving a medicinal substance or a salt thereof in a fattyacid-based ionic liquid to form a composite ionic composition of the medicinalsubstance. The external preparation composition can be used as a liquid preparation,an ointment, a cream, a plaster or the like, and enables to provide a preparationhaving excellent transdermal absorbability.