A pyridine derivative represented by the following formula (I) or a pharmaceutically acceptable salt thereof, or a solvate thereof: ** (See formula) ** wherein: A represents a single bond, an oxygen atom, an atom sulfur, NH or CH2； R1 represents a nitrogen atom or CH； one from X1 to X5 represents a nitrogen atom and the remaining four represent CR2； each R2 independently represents a hydrogen atom, an alkyl group having 1 to 6 carbon atoms, an alkenyl group having 2 to 6 carbon atoms, an alkynyl group having 2 to 6 carbon atoms, an atom of halogen, a trifluoromethyl group, a difluoromethyl group, a cyano group, an alkylcarbonyl group having 2 to 7 carbon atoms, an alkylsulfonyl group having 1 to 6 carbon atoms, a nitro group, an amino group, a dialkylamino group having 1 to 6 carbon atoms that may optionally form a ring, a formyl group, a hydroxyl group, an alkoxy group having 1 to 6 carbon atoms (which may be optionally substituted with one or more of one hydroxyl group, a phenyl group, a cyclohexyl group and a halogen atom), an alkylthio group having 1 to 6 carbon atoms, a phenyl group (which may be optionally substituted with one or more of an alkyl group having 1 to 6 át carbon atoms, an alkoxy group having 1 to 6 carbon atoms and a halogen atom), or a phenoxy group (which may be optionally substituted with one or more of an alkyl group having 1 to 6 carbon atoms , an alkoxy group having 1 to 6 carbon atoms and a halogen atom), with the proviso that when two of CR2 are adjacent, the two of R2 can be optionally linked to form a ring； R3 represents a hydrogen atom, an alkyl group having 1 to 6 carbon atoms (which may be optionally substituted with one or more of a hydroxyl group, an amino group, a dialkylamino group having 1 to 6 carbon atoms which may optionally form a ring, an imidazole ring, a pyrazole ring, a pyrrolidine ring, a piperidine ring, a morpholine ring and a piperazine ring (which may be optionally substituted with one or more of an alkyl group havi
