A compound represented by the formula: wherein R 1 is a C 1-6 alkyl group; R 2 is a C 1-6 alkyl group optionally having 1 to 3 substituents selected from (a) a hydroxy group, (b) a halogen atom, (c) a C 1-6 alkoxy group, (d) a C 1-6 alkyl-carbonyloxy group, (e) an aromatic heterocyclic group optionally having 1 to 3 halogen atoms, (f) a C 3-10 cycloalkyl group, and (g) a cyclic amino group optionally having an oxo group, a 3- to 10-membered heterocyclic group optionally having 1 to 3 substituents selected from a C 1-6 alkyl group and an oxo group, a carboxy group, a C 1-6 alkoxy-carbonyl group optionally having 1 to 3 substituents selected from a nonaromatic heterocyclic group optionally having 1 to 3 substituents selected from a C 1-6 alkyl group and oxo group, a C 1-6 alkyl-carbonyl group, or a group represented by the formula: -CO-NR'R" wherein R' and R" are each a hydrogen atom, or R' and R" form, together with the nitrogen atom bonded thereto, a nitrogen-containing heterocycle optionally having 1 to 3 substituents selected from halogen atom(s); X is a hydrogen atom; a C 1-6 alkyl group optionally substituted by 1 to 3 substituents selected from (a) a halogen atom, (b) a hydroxy group, (c) a C 1-6 alkoxy group optionally having a C 1-6 alkoxy group or a halogen atom, (d) a C 1-6 alkylthio group, (e) an aryl group, (f) an aryloxy group optionally having a C 1-6 alkoxy group or a halogen atom, and (g) a heteroaryl group; or a C 3-10 cycloalkyl group; and ring A is a ring represented by the formula or a salt thereof. R 1 is a C 1-6 alkyl group; R 2 is a C 1-6 alkyl group optionally having 1 to 3 substituents selected from (a) a hydroxy group, (b) a halogen atom, (c) a C 1-6 alkoxy group, (d) a C 1-6 alkyl-carbonyloxy group, (e) an aromatic heterocyclic group optionally having 1 to 3 halogen atoms, (f) a C 3-10 cycloalkyl group, and (g) a cyclic amino group optionally having an oxo group, a 3- to 10-membered heterocyclic group optionally having 1 to 3 substituents selected from a C 1-6 alkyl group and an oxo group, a carboxy group, a C 1-6 alkoxy-carbonyl group optionally having 1 to 3 substituents selected from a nonaromatic heterocyclic group optionally having 1 to 3 substituents selected from a C 1-6 alkyl group and oxo group, a C 1-6 alkyl-carbonyl group, or a group represented by the formula: -CO-NR'R" a C 1-6 alkyl group optionally having 1 to 3 substituents selected from (a) a hydroxy group, (b) a halogen atom, (c) a C 1-6 alkoxy group, (d) a C 1-6 alkyl-carbonyloxy group, (e) an aromatic heterocyclic group optionally having 1 to 3 halogen atoms, (f) a C 3-10 cycloalkyl group, and (g) a cyclic amino group optionally having an oxo group, (a) a hydroxy group, (b) a halogen atom, (c) a C 1-6 alkoxy group, (d) a C 1-6 alkyl-carbonyloxy group, (e) an aromatic heterocyclic group optionally having 1 to 3 halogen atoms, (f) a C 3-10 cycloalkyl group, and (g) a cyclic amino group optionally having an oxo group, a 3- to 10-membered heterocyclic group optionally having 1 to 3 substituents selected from a C 1-6 alkyl group and an oxo group, a carboxy group, a C 1-6 alkoxy-carbonyl group optionally having 1 to 3 substituents selected from a nonaromatic heterocyclic group optionally having 1 to 3 substituents selected from a C 1-6 alkyl group and oxo group, a C 1-6 alkyl-carbonyl group, or a group represented by the formula: -CO-NR'R" wherein R' and R" are each a hydrogen atom, or R' and R" form, together with the nitrogen atom bonded thereto, a nitrogen-containing heterocycle optionally having 1 to 3 substituents selected from halogen atom(s); X is a hydrogen atom; a C 1-6 alkyl group optionally substituted by 1 to 3 substituents selected from (a) a halogen atom, (b) a hydroxy group, (c) a C 1-6 alkoxy group optionally having a C 1-6 alkoxy group or a halogen atom, (d) a C 1-6 alkylthio group, (e) an aryl group, (f) an aryloxy group optionally having a C 1-6 alkoxy group or a halogen atom, and (g) a heteroaryl group; or a C 3-10 cycloalkyl group; and a hydrogen atom; a C 1-6 alkyl group optionally substituted by 1 to 3 substituents selected from (a) a halogen atom, (b) a hydroxy group, (c) a C 1-6 alkoxy group optionally having a C 1-6 alkoxy group or a halogen atom, (d) a C 1-6 alkylthio group, (e) an aryl group, (f) an aryloxy group optionally having a C 1-6 alkoxy group or a halogen atom, and (g) a heteroaryl group; or (a) a halogen atom, (b) a hydroxy group, (c) a C 1-6 alkoxy group optionally having a C 1-6 alkoxy group or a halogen atom, (d) a C 1-6 alkylthio group, (e) an aryl group, (f) an aryloxy group optionally having a C 1-6 alkoxy group or a halogen atom, and (g) a heteroaryl group; or a C 3-10 cycloalkyl group; and ring A is a ring represented by the formula or a salt thereof.