The present invention relates to use of peptide to which G-protein coupled receptor protein recognizes as a ligand. Since the ligand polypeptide of the present invention has a stimulating action on oxytocin secretion, it is useful as a drug for ameliorating, preserving or treating various diseases related to oxytocin secretion such as uterine inertia, atonic hemorrhage, placental expulsion, subinvolution and the like.