This invention provides a method for detecting cognitive disfunction diseases including mild cognitive impairment and Alzheimer's disease using a protein and a peptide of the protein different in the presence level in subjects having a normal cognitive function and patients suffering from cognitive disfunction diseases and a biomarker for detecting cognitive disfunction diseases including mild cognitive impairment and Alzheimer's disease containing the protein and the peptide. This invention is a biomarker for diagnosing cognitive disfunction diseases containing a prothrombin precursor protein of SEQ ID NO: 1 or a peptide THRB containing the amino acid sequence represented by SEQ ID NO: 2 which is a peptide of the protein, a diagnosis method for cognitive disfunction diseases using the biomarker, an antigen peptide represented by SEQ ID NO: 3 for creating a THRB peptide specific antibody to be used in the diagnosis method, and a cognitive disfunction disease diagnosis kit containing the THRB peptide specific antibody.