A compound represented by the formula or a pharmaceutically acceptable salt thereof: wherein R 1 represents OR, NRR' which may form a ring or SR, in which R and R' independently represent a hydrogen atom, a linear, cyclic or branched alkyl group having 1 to 8 carbon atoms optionally substituted with one or a plurality of linear, cyclic or branched alkoxy groups having 1 to 8 carbon atoms, halogen atoms or hydroxyl groups, an aryl group optionally substituted with one or a plurality of linear, cyclic or branched alkyl groups having 1 to 8 carbon atoms, linear, cyclic or branched alkoxy groups having 1 to 8 carbon atoms or halogen atoms, or a heteroaryl group optionally substituted with one or a plurality of linear, cyclic or branched alkyl groups having 1 to 8 carbon atoms, a linear, cyclic or branched alkoxy groups having 1 to 8 carbon atoms or a halogen atom, R 2 represents a hydrogen atom, or a linear, cyclic or branched alkyl group having 1 to 8 carbon atoms, and X 1, X 2 and X 3 are independently CR 3 or a nitrogen atom, or X 1 is CR 3 or a nitrogen atom, and X 2 and X 3 together form a benzene ring, in which R 3 is a hydrogen atom, or a linear, cyclic or branched alkyl group having 1 to 8 carbon atoms, wherein "an aryl group" means a monocyclic or bicyclic aromatic hydrocarbon group having 6 to 10 carbon atoms, and wherein "a heteroaryl group" means a pyridyl group, pyrazyl group, pyrimidyl group, furyl group, thienyl group, isoxazolyl group, isothiazolyl group, benzofuranyl group, benzothienyl group, benzothiazolyl group, benzoimidazolyl group, benzooxazolyl group, pyranyl group, imidazolyl group, oxazolyl group, thiazolyl group, triazinyl group, triazolyl group, benzoxazolyl group, or benzoisoxazolyl group. wherein R 1 represents OR, NRR' which may form a ring or SR, in which R and R' independently represent a hydrogen atom, a linear, cyclic or branched alkyl group having 1 to 8 carbon atoms optionally substituted with one or a plurality of linear, cyclic or branched alkoxy groups having 1 to 8 carbon atoms, halogen atoms or hydroxyl groups, an aryl group optionally substituted with one or a plurality of linear, cyclic or branched alkyl groups having 1 to 8 carbon atoms, linear, cyclic or branched alkoxy groups having 1 to 8 carbon atoms or halogen atoms, or a heteroaryl group optionally substituted with one or a plurality of linear, cyclic or branched alkyl groups having 1 to 8 carbon atoms, a linear, cyclic or branched alkoxy groups having 1 to 8 carbon atoms or a halogen atom, R 2 represents a hydrogen atom, or a linear, cyclic or branched alkyl group having 1 to 8 carbon atoms, and X 1, X 2 and X 3 are independently CR 3 or a nitrogen atom, or X 1 is CR 3 or a nitrogen atom, and X 2 and X 3 together form a benzene ring, in which R 3 is a hydrogen atom, or a linear, cyclic or branched alkyl group having 1 to 8 carbon atoms, wherein "an aryl group" means a monocyclic or bicyclic aromatic hydrocarbon group having 6 to 10 carbon atoms, and wherein "a heteroaryl group" means a pyridyl group, pyrazyl group, pyrimidyl group, furyl group, thienyl group, isoxazolyl group, isothiazolyl group, benzofuranyl group, benzothienyl group, benzothiazolyl group, benzoimidazolyl group, benzooxazolyl group, pyranyl group, imidazolyl group, oxazolyl group, thiazolyl group, triazinyl group, triazolyl group, benzoxazolyl group, or benzoisoxazolyl group.