Provided herewith is a therapeutic agent or prophylactic agent comprising an antibody for use in a method of reducing tau protein that has been phosphorylated on an amino acid residue present at position 413 of the tau protein represented by SEQ ID NO: 1 in the brain, wherein the antibody participates in antigen-antibody reaction with a peptide comprising at least 8 contiguous amino acids within 410-421 of SEQ ID NO: 1, the amino acid at position 413 being phosphorylated； and wherein the binding of the antibody to the tau protein phosphorylated at the amino acid residue corresponding to Ser413 of SEQ ID NO: 1 is at least 10 time stronger than the binding to a tau protein that has not been phosphorylated at said Ser413.
