A 50-year-old man with a family history of stroke and depression slowly developed brain lesions. Magnetic resonance imaging revealed hyperintense lesions in the diffuse white matter, external capsules, and temporal poles on T2-weighted imaging. A heterozygous mutation c.3879C>G in exon 24 of the NOTCH3 gene (p.Cys1293Trp) was detected, confirming a diagnosis of cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy (CADASIL). Exon 24 mutations are rather rare and this represents the first Japanese case of CADASIL.