Provided is a medicament or a method for treating or preventing a corneal endothelial condition, disorder or disease due to a transforming growth factor-β (TGF-β) signal in corneal endothelial cells using a p38 MAP kinase inhibitor (in particular AKP-001), such as a pyrimidinyl isoxazole derivative or a pyridyl isoxazole derivative. In a preferred embodiment, the corneal endothelial condition, disorder or disease due to the TGF-β signal is Fuchs' corneal endothelial dystrophy.