Biomarker for mental disorders including cognitive disorders, and method using said biomarker to detect mental disorders including cognitive disorders

Methods are provided that detect cognitive impairment including mild cognitive impairment and Alzheimer disease by using a protein or its partial peptide that differs in presence or absence. Novel biomarkers are also provided for cognitive impairment and non-psychiatric disease, as well as methods for detecting cognitive impairment using such biomarkers. Specifically, a biomarker for diagnosis is provided that comprises a protein fragment or peptide of not less than 5 amino acid residues arising from at least one protein or peptide selected from the group of proteins consisting of an amino acid sequence expressed by SEQ ID NO: 1, 3, 6, 8, 10, 13, 15, 18, or 20 and selected from the group of partial peptide in these proteins consisting of an amino acid sequence expressed by SEQ ID NO: 2, 4, 5, 7, 9, 11, 12, 14, 16, 17, 19, or 21.