A medical instrument for transdermally administering a medicine, in which an ionic medicine or an ionic medicine-containing substance is sealed in a bag made of an ion-exchange membrane. The medical instrument for transdermally administering the medicine is favorably used for executing the iontophoresis. By using this medical instrument, an iontophoresis device is realized in a simple structure, inexpensively and in a small size, which is suited for being carried yet making it easy to exchange the ionic medicine.