The present invention relates to a sustained-release formulation comprising ametastin derivative and a lactic polymer having aweight average molecular weight of about 5,000 to about 40,000 or a saltthereof. The sustained-release formulationof the present invention slowly and stably releases compound (I) or a saltthereof over a long period of time and exerts medicinaleffects of compound (I) or a salt thereof over a long period of time.Furthermore, the sustained-release formulation of the presentinvention, which improves patient's convenience by reducing frequency ofadministration, is an excellent formulation as a clinicalmedicine.
