NUT midline carcinoma (NMC) is a recently described, undifferentiated carcinoma with specific NUT gene rearrangement, which often involves midline organs such as the nasal cavity, paranasal sinuses, mediastinum, or intrathoracic organs. It was previously considered a disease of children or young adults, but middle-aged or elderly patients have subsequently been seen. Here, the authors report the case of a 54-year-old woman who presented with a left-nasal-cavity mass and diplopia. The tumor enlarged rapidly and extended to the left orbital cavity and brain base despite chemotherapy and radiotherapy. Pathological examination of the resected tumor showed an undifferentiated carcinoma with occasional abrupt keratinizing squamous differentiation. Immunohistochemical analysis with an antibody to NUT revealed that most of the tumor cells were positive. BRD4-NUT gene fusion was demonstrated by fluorescence in situ hybridization, confirming the diagnosis of NMC. This case emphasizes the importance of considering NMC in the differential diagnosis in older adults.