[PROBLEMS] To provide a protective agent for a retinal nerve or an optic nerve,which exhibits its efficacy through a mechanism that is different from that fora conventional therapeutic agent and can be ingested over a long period. [MEANSFOR SOLVING PROBLEMS] Disclosed is protective agent for a retinal nerve or anoptic nerve, which comprises a compound having an inhibitory activity on an aldosereductase, such as a compound represented by the general formula below, as anactive ingredient. Preferably, the compound is (2S,4S)-6- fluoro-2',5'-dioxospiro[chromane-4,4'-imidazolidine]-2-carboxamide. wherein X represents a halogen atom ora hydrogen atom； Rand R independently represent a hydrogen atom or a Calkyl group which may be substituted, or R and R togetherwith a nitrogen atom located at the base of R and R or additionallytogether with other nitrogen atom or an oxygen atom may form a 5- to 6-memberedheterocyclic ring.
