We report a 39-year-old woman who presented with only dizziness and vertigo for 2 months. Neurological examination revealed no abnormalities except for hypereflexia on the left side extremities. Neurootological examination revealed no abnormalities. MRI of the brain demonstrated patchy hyperintensity areas on FLAIR images in the periventricular white matter and external capsule. Her grandmother had cerebral infarction and her father is suffering from multi-infarct dementia. Her second older sister who similarly had dizziness and vertigo demonstrated similar MRI findings characterized by patchy hyperintensity areas in the white matter and external capsule even though she had no risk factors for atherosclerosis. Her third older sister also had dizziness and vertigo and had patchy hyperintensity areas in the white matter in her brain MRI even though she had no risk factors for atherosclerosis. Based on this family history, we suspected that she had cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy (CADASIL). Mutational analysis of Notch3 gene disclosed a novel missense mutation substituting arginine for cysteine at codon 206 (C206R) in exon 4 of the Notch3 gene, confirming the diagnosis of CADASIL. Interestingly, similar dizziness and vertigo were present not only in the patient, but also in the other two sisters who had the same gene mutation as the patient. This report supports the idea that the external capsule lesion is one of the signs suggestive of CADASIL as a diagnosis.