The present invention aims at providing an antibody, by which metastin or its derivative can be quantified specifically with a high sensitivity, a method of detecting/quantifying metastin or its derivative using the antibody, and a diagnostic agent (e.g., a diagnostic for pregnancy) the same. Specifically, an antibody capable of specifically reacting with a partial peptide at the N-terminus or C-terminus of a polypeptide having the amino acid sequence represented by SEQ ID NO: 1 or its derivative, and a method of detecting/quantifying metastin or its derivative using the antibody as well as a diagnostic agent using the same.