An external plaster using an aqueous hot melted type adhesive base material stably containing water-soluble drug having good drug releasability, so that bioavailability of the drug is enhanced, is provided. The aqueous hot melted type adhesive base material used in the external plaster comprising as essential components a styrene-isoprene-styrene block copolymer, an adhesive resin, an antioxidant, lanolin, and water.