A transdermal drug administration device which comprises a patch that is provided with: a donor gel in which a medical agent is sealed a reference gel a first electrode that is connected to the donor gel and a second electrode that is connected to the reference gel. When the patch is applied to the skin, a direct current is applied between the first electrode and the second electrode at a predetermined cycle and an alternating current is applied between the first electrode and the second electrode during the periods when a direct current is not applied, thereby obtaining the capacitance. The application state of the patch is determined based on the thus-obtained capacitance.
