The present invention provides an antibody which specifically recognizes a CNS tau protein but not a peripheral tau protein. More specifically, the present invention provides an antibody obtainable by using a polypeptide comprising an amino acid sequence of a connective portion between the amino acid sequence encoded by Exon 4 of a gene encoding a tau protein and the amino acid sequence encoded by Exon 5 thereof as an epitope specific to the isoform of tau protein predominantly existing in central nervous tissues. The present invention further provides a method of detecting Alzheimer's disease and a reagent kit using the antibody.