The invention provides stable metastin derivatives having excellent biological activities (a cancer metastasis suppressing activity, a cancer growth suppressing activity, etc.). By modifying the constituent amino acids of metastin with specific modifying groups, metastin derivatives having more improved blood stability, etc. than native metastin and showing excellent cancer metastasis suppressing activity or cancer growth suppressing activity have been found. Furthermore, it has been found that these metastin derivatives exhibit effects of suppressing gonadotropic hormone secretion, suppressing sex hormone secretion, etc., which are wholly different from the effects heretofore known.