The present invention provides a protective agent for retinal nerve or optic nerve which exerts its effect through a different mechanism from that of conventional therapeutic agents and can be taken for long periods. The present invention is the protective agent for retinal nerve or optic nerve which comprises a compound with aldose reductase inhibiting activity such as the compound represented by the following general formula as an active ingredient. A preferable compound thereof is (2S,4S)-6-fluoro-2',5'-dioxospiro[chroman-4,4'-imidazolidine]-2-carboxamide:wherein X represents a halogen atom or a hydrogen atom, R1 and R2 independently represent a hydrogen atom or an optionally substituted C1 to C6 alkyl group, or R1 and R2, together with a nitrogen atom bound thereto, or optionally another nitrogen atom or an oxygen atom, are combined to form a 5- to 6-membered heterocycle.
