A 72-year-old man presented to our neurology clinic with a history of progressive memory decline for 1 year, followed by acute cognitive deterioration for 1 month. A score of 21/30 on the Mini-Mental State Examination was recorded, and the clinical dementia rating (CDR) was 1. Brain MRI showed generalized brain atrophy, white matter hyperintensity at bilateral subcortical white matter, and no microbleed on susceptibility-weighted imaging; diffusion-weighted imaging (DWI) revealed only equivocal hyperintense signal at the corticomedullary junction, which was deemed to be not significant at the time. Amyloid imaging with PiB PET scan showed increased uptake, mainly in the prefrontal cortices. Alzheimer’s disease was tentatively diagnosed, and he started taking memantine. However, his cognitive function gradually deteriorated, and the CDR became 4 in 1 year. Episodic encephalopathy also developed. Follow-up brain MRI showed progressive DWI hyperintensity at corticomedullary junction, characteristic of NIID. Skin biopsy demonstrated the presence of ubiquitin staining-positive intranuclear inclusions in the sweat gland epithelial cells and vascular smooth muscle cells. Genetic analysis revealed abnormal GGC repeat expansions in the 5’UTR of the NOTCH2NLC gene, which is pathogenic for NIID.