To provide a novel furanone derivative, and a medicine including the same. The furanone derivative is represented by the formula (I): wherein A represents —COOR1 or a hydrogen atom; R1 represents a hydrogen atom, an optionally substituted hydrocarbon group, or an optionally substituted heterocycle; R2 and R3 are the same or different and each independently represent a hydrogen atom, an optionally substituted hydrocarbon group, an optionally substituted phenyl group, an optionally substituted heterocycle, an optionally substituted heterocyclic fused ring, or an optionally substituted amino group; or alternatively, R2 and R3, taken together with the nitrogen atom to which they are attached, may form an optionally substituted heterocycle or an optionally substituted heterocyclic fused ring; and R4 represents a hydrogen atom or a halogen atom; with the proviso that when A represents —COOR1, R2 and R3 are not optionally substituted amino groups at the same time, and when A represents a hydrogen atom, R3 represents a hydrogen atom.