We report a 38-year-old Japanese woman who had cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy (CADASIL) with a novel mutation (TGT to TAT) at nucleotide position 1241 (C388Y) in exon 7 of the Notch3 gene (NOTCH3). Immunostaining of a skin biopsy with a Notch3 monoclonal antibody is a beneficial method for the screening of CADASIL, particularly in the case of rare mutations outside the mutation hotspots in NOTCH3 as shown in this patient.