A compound represented by the formula: or a salt thereof; wherein: the group represented by: is a group represented by: X a1 is CH or a nitrogen atom; X a2 is: CR a wherein R a is: (a) a hydrogen atom, (b) a halogen atom, or (c) a C 1-6 alkyl group, or a nitrogen atom; X a3 is: CR a wherein R a is: (a) a hydrogen atom, or (b) a C 1-6 alkyl group, or a nitrogen atom; X a4 is: CR a wherein R a is: (a) a hydrogen atom, or (b) a C 1-6 alkyl group, or a nitrogen atom; and X a5 is: CR a wherein R a is: (a) a hydrogen atom, (b) a C 1-6 alkyl group optionally substituted by 1 to 3 substituents selected from a halogen atom and a C 1-6 alkoxy group, (c) a C 1-6 alkoxy group, or (d) a C 3-10 cycloalkyl group, or a nitrogen atom; provided that when X a5 is CR a, then at least one of X a1, X a2 and X a3 is a nitrogen atom; X b1 is CH or a nitrogen atom; X b2 is CR b wherein R b is: (a) a hydrogen atom, or (b) a C 1-6 alkyl group; X b3 is a nitrogen atom optionally substituted by a C 1-6 alkyl group optionally substituted by 1 to 3 halogen atoms; and X b4 is CR b wherein R b is: (a) a hydrogen atom, or (b) a C 1-6 alkyl group; R 1 and R 2 are both hydrogen atoms; and R 3 is a group represented by: R 4 is: a C 1-6 alkyl group optionally substituted by 1 to 3 halogen atoms, a C 1-6 alkoxy group optionally substituted by 1 to 3 halogen atoms, a C 3-10 cycloalkyloxy group, or a halogen atom; Y is a nitrogen atom or CR 6 wherein R 6 is a hydrogen atom or a halogen atom; and Ring C 1 is a benzene ring or a pyridine ring, each of which is optionally further substituted by one halogen atom; R 5 is a C 1-6 alkyl group optionally substituted by 1 to 3 halogen atoms; and Ring C 2 is a thiophene ring substituted by only R 5. the group represented by: is a group represented by: X a1 is CH or a nitrogen atom; X a2 is: CR a wherein R a is: (a) a hydrogen atom, (b) a halogen atom, or (c) a C 1-6 alkyl group, or a nitrogen atom; CR a wherein R a is: (a) a hydrogen atom, (b) a halogen atom, or (c) a C 1-6 alkyl group, or (a) a hydrogen atom, (b) a halogen atom, or (c) a C 1-6 alkyl group, or a nitrogen atom; X a3 is: CR a wherein R a is: (a) a hydrogen atom, or (b) a C 1-6 alkyl group, or a nitrogen atom; CR a wherein R a is: (a) a hydrogen atom, or (b) a C 1-6 alkyl group, or (a) a hydrogen atom, or (b) a C 1-6 alkyl group, or a nitrogen atom; X a4 is: CR a wherein R a is: (a) a hydrogen atom, or (b) a C 1-6 alkyl group, or a nitrogen atom; and CR a wherein R a is: (a) a hydrogen atom, or (b) a C 1-6 alkyl group, or (a) a hydrogen atom, or (b) a C 1-6 alkyl group, or a nitrogen atom; and X a5 is: CR a wherein R a is: (a) a hydrogen atom, (b) a C 1-6 alkyl group optionally substituted by 1 to 3 substituents selected from a halogen atom and a C 1-6 alkoxy group, (c) a C 1-6 alkoxy group, or (d) a C 3-10 cycloalkyl group, or a nitrogen atom; CR a wherein R a is: (a) a hydrogen atom, (b) a C 1-6 alkyl group optionally substituted by 1 to 3 substituents selected from a halogen atom and a C 1-6 alkoxy group, (c) a C 1-6 alkoxy group, or (d) a C 3-10 cycloalkyl group, or (a) a hydrogen atom, (b) a C 1-6 alkyl group optionally substituted by 1 to 3 substituents selected from a halogen atom and a C 1-6 alkoxy group, (c) a C 1-6 alkoxy group, or (d) a C 3-10 cycloalkyl group, or a nitrogen atom; provided that when X a5 is CR a, then at least one of X a1, X a2 and X a3 is a nitrogen atom; X b1 is CH or a nitrogen atom; X b2 is CR b wherein R b is: (a) a hydrogen atom, or (b) a C 1-6 alkyl group; (a) a hydrogen atom, or (b) a C 1-6 alkyl group; X b3 is a nitrogen atom optionally substituted by a C 1-6 alkyl group optionally substituted by 1 to 3 halogen atoms; and X b4 is CR b wherein R b is: (a) a hydrogen atom, or (b) a C 1-6 alkyl group; (a) a hydrogen atom, or (b) a C 1-6 alkyl group; R 1 and R 2 are both hydrogen atoms; and R 3 is a group represented by: R 4 is: a C 1-6 alkyl group optionally substituted by 1 to 3 halogen atoms, a C 1-6 alkoxy group optionally substituted by 1 to 3 halogen atoms, a C 3-10 cycloalkyloxy group, or a halogen atom; a C 1-6 alkyl group optionally substituted by 1 to 3 halogen atoms, a C 1-6 alkoxy group optionally substituted by 1 to 3 halogen atoms, a C 3-10 cycloalkyloxy group, or a halogen atom; Y is a nitrogen atom or CR 6 wherein R 6 is a hydrogen atom or a halogen atom; and Ring C 1 is a benzene ring or a pyridine ring, each of which is optionally further substituted by one halogen atom; R 5 is a C 1-6 alkyl group optionally substituted by 1 to 3 halogen atoms; and Ring C 2 is a thiophene ring substituted by only R 5.