1. A protective agent for a retinal neuronal cell comprising as an active ingredient a compound represented by the following general formula (1) or a salt thereof: the ring X represents a benzene ring or a pyridine ring; R 1 and R 2 are the same or different and represent a hydrogen atom or an alkyl group; R 1 and R 2 may be joined together to form a cycloalkane ring; R 3 represents a hydroxy group, an alkoxy group, an aryloxy group, a cycloalkyl group, an alkylamino group, an arylamino group, a nitro group, a cyano group or a monocyclic heterocycle; and a cyclic moiety of the benzene ring, pyridine ring, cycloalkane ring, aryloxy group, cycloalkyl group, arylamino group or monocyclic heterocycle defined above may have one or plural substituents selected from a halogen atom, an alkoxy group and an alkyl group, and an alkyl moiety of the alkyl group, alkoxy group or alkylamino group defined above may have one or plural substituents selected from a halogen atom and an alkoxy group.