PROBLEM TO BE SOLVED: To provide a novel dementia therapeutic agent or preventive agent.SOLUTION: Dementia therapeutic agents, preventive agents, or the like contains, as an active ingredient, antibodies that specifically exhibit an antigen-antibody reaction to tau proteins of which the area around Ser413 of SEQ ID NO: 1 is phosphorylated; and peptides of which the area around Ser413 is phosphorylated.SELECTED DRAWING: None