Disclosed is a stable metastin derivative having excellent biological activities (e.g., a cancer metastasis inhibiting activity, a cancer proliferation inhibiting activity, a gonadotropin secretion promoting activity, a sex hormone secretion promoting activity). A metastin derivative produced by substituting a constituent amino acid in metastin can be improved in stability in the blood and can have an excellent cancer metastasis inhibiting activity, an excellent cancer proliferation inhibiting activity, an excellent gonadotropin secretion inhibiting activity, an excellent sex hormone secretion inhibiting activity and the like by binding the metastin derivative to polyethylene glycol.
