The present invention provides a novel therapeutic agent or prophylactic agent for dementia. The present invention provides a therapeutic agent, prophylactic agent, or the like for dementia that contains a substance such as the following 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.