The administration efficiency of a drug may be improved by establishing compatibility between the suppression of the release of a biological counter ion from a living body's biological interface and the maintenance of a good state of contact between each of a working electrode structure and a nonworking electrode structure and the biological interface. Coating films each composed of an external preparation containing: a hydrophilic polymer matrix agent; and an ion-exchange resin dispersed in the hydrophilic polymer matrix agent, or coating films each composed of an external preparation containing a hydrophilic polymer having an ion-exchange function are formed on the biological interface, and iontophoresis is performed through the coating films.