Provided are: a depot preparation comprising tafluprost which can form depots easily and can maintain the depot-formed state for a long period so that tafluprost can be sustainably released over a long period of time after in vivo administration thereof； and a method for stabilizing tafluprost. The present invention pertains to a depot preparation comprising tafluprost and a trialkyl citrate and/or an acetyltrialkyl citrate, wherein the alkyl groups in the trialkyl citrate and the alkyl groups in the acetyltrialkyl citrate are either the same or different and each of these alkyl groups has 3-5 carbon atoms.本發明提供一種他氟前列素(Tafluprost)之緩釋劑及將他氟前列素穩定化之方法，該他氟前列素之緩釋劑由於容易形成儲存庫，且長時間維持儲存庫狀態，故而投予至體內之後長時間地緩釋他氟前列素。本發明係關於一種緩釋劑，其含有他氟前列素與檸檬酸三烷基酯及/或乙醯檸檬酸三烷基酯，且上述檸檬酸三烷基酯及上述乙醯檸檬酸三烷基酯所分別具有之烷基相同或不同，碳原子數為3~5。
