[PURPOSE] The invention provides a novel therapeutic agent or prophylactic agent for cognitive disorders. [SOLUTION MEANS] The invention provides an antibody that participates in antigen-antibody reaction specifically with tau protein that has been phosphorylated in the vicinity of Ser413 of SEQ ID NO: 1, and a therapeutic agent or prophylactic agent for cognitive disorders comprising as an active ingredient a peptide that has been phosphorylated in the vicinity of Ser413.