Abstract: This invention provides novel 2-(N-cyanoimino)thiazolidin-4-one derivatives represented by formula I or a pharmaceutically acceptable salt or solvate thereof: wherein ring A represents a benzene ring, a condensed ring or a heterocyclic ring, each of which may be substituted by one or more substituents selected from a straight or branched C1-C4 alkyl group, a haloalkyl group, a halogen atom or —OR5, R1 represents a single bond, an oxygen atom, a sulfur atom, a methyne group, a straight or branched C1-C4 alkylene or alkenylene group optionally substituted by a phenyl group, R6—X, X—R6, X—R6—X, R6—X—R6, —C(═O)—NR7— or —NR7—C(═O)—, R2 and R3 are the same or different and each represents a hydrogen atom, a C1-C4 alkyl group, —OR8 or a halogen atom, R4 represents a hydrogen atom or a C1-C4 alkyl group, R5 represents a hydrogen atom or a C1-C4 alkyl group, R6 represents a straight or branched C1-C4 al