Provided is a device for treating tinea unguium which allows unrestricted treatment even for patients with basic diseases such as liver diseases, pregnant women and lactating patients and by which interindividual difference in the treatment can be avoided and antifungal agent for external use can be transdermally absorbed from the nail surface to the deep part of the nail economically, safely and efficiently within a short period of time without any side effects. A dome-shaped applicator, which is formed of a flexible material, which has an open part provided with a flange for adhesion and which is formed to have a larger film thickness in the periphery adjacent to the open part than the film thickness in other parts, is hermetically bonded to a surface of a nail. Then, the liquid antifungal agent for external use is fed into the dome-shaped applicator under pressure using an injector and is allowed to be efficiently penetrated into the deep part of the nail using positive pressure.
