Provided is a simple device for treating trichophytosis 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 an 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 comprising a flexible material, in which an edge part for adhesion is formed in an open part of the dome-shaped applicator and which is molded in such a manner that the film thickness in the periphery of the open part is larger than the film thickness in other parts, is brought into air-tight contact with the surface of a nail. Then, a liquid antifungal agent for external use is pressed from outside into the dome-shaped applicator with the use of a press-in device or the like. Thus, the antifungal agent for external use is efficiently penetrated into the deep part of the nail by using positive pressure.
