An (aza)indolizine derivative represented by the formula: wherein 0 to 2 of X 1, X 2, X 3 and X 4 are a nitrogen atom and the others are CR 1; one of T 1 and T 2 represents cyano and the other represents a group represented by the formula: and with the proviso that when T 1 is cyano, at least one of X 1 to X 4 is a nitrogen atom; R 1 independently represents any one of the following to: a hydrogen atom; a halogen atom; a hydroxy group; amino; cyano; carboxy; C 1-6 alkyl, C 2-6 alkenyl, C 2-6 alkynyl, C 1-6 alkoxy, mono(di) C 1-6 alkylamino, C 2-7 acyl, C 2-7 acylamino, mono(di)C 1-6 alkylcarbamoyl, C 1-6 alkylsulfonyl, C 1-6 alkylsulfonylamino, mono(di)C 1-6 alkylsulfamoyl, C 1-6 alkylthio, C 3-8 cycloalkyl, 3 to 8-membered heterocycloalkyl, C 5-8 cycloalkenyl, 5 to 8-menbered heterocycloalkenyl, C 3-8 cycloalkyloxy, C 3-8 cycloalkylamino, C 6 aryl, 5 or 6-membered heteroary, C 6 aryloxy, C 6 arylamino, C 6 arylcarbonyl or C 6 arylcarbonylamino each of which may have any group selected from substituent group G; substituent group G consists of a fluorine atom, a chlorine atom, a hydroxy group, amino, carboxy, carbamoyl, cyano, C 1-6 alkyl, C 1-6 alkoxy and mono(di)C 1-6 alkylamino; ring U represents a benzene ring or 5 or 6-membered heteroaryl; m represents an integral number from 0 to 2; and R 2 represents a fluorine atom, a hydroxy group, amino, methyl or trifluoromethyl, and when m is 2, two R 2 are optionally different from each other, or a prodrug thereof, or a pharmaceutically acceptable salt thereof, in which "prodrug" means a compound wherein any one or more groups selected from a hydroxyl and a carboxy groups of a compound represented by the above formula is substituted by a group forming a prodrug, in which the group forming a prodrug is selected from the group consisting of: (in a hydroxyl group), C 1-6 alkyl-CO-, C 6-10 aryl-CO-, C 1-6 alkyl-O-C 1-6 alkylene-CO-, C 1-6 alkyl-OCO-C 1-6 alkylene-CO-, C 1-6 alkyl-OCO-, C 1-6 alkyl-O-C 1-6 alkylene-OCO-, C 1-6 alkyl-COO-C 1-6 alkylene, C 1-6 alkyl-OCOO-C 1-6 alkylene, C 3-8 cycloalkyl-OCOO-C 1-6 alkylene, and an ester or an amide with an amino acid; (in a carboxy group), C 1-6 alkyl, C 1-6 alkyl-COO-C 1-6 alkylene, C 1-6 alkyl-OCOO-C 1-6 alkylene, and C 3-8 cycloalkyl-OCOO-C 1-6 alkylene. 0 to 2 of X 1, X 2, X 3 and X 4 are a nitrogen atom and the others are CR 1; one of T 1 and T 2 represents cyano and the other represents a group represented by the formula: and with the proviso that when T 1 is cyano, at least one of X 1 to X 4 is a nitrogen atom; R 1 independently represents any one of the following to: a hydrogen atom; a halogen atom; a hydroxy group; amino; cyano; carboxy; C 1-6 alkyl, C 2-6 alkenyl, C 2-6 alkynyl, C 1-6 alkoxy, mono(di) C 1-6 alkylamino, C 2-7 acyl, C 2-7 acylamino, mono(di)C 1-6 alkylcarbamoyl, C 1-6 alkylsulfonyl, C 1-6 alkylsulfonylamino, mono(di)C 1-6 alkylsulfamoyl, C 1-6 alkylthio, C 3-8 cycloalkyl, 3 to 8-membered heterocycloalkyl, C 5-8 cycloalkenyl, 5 to 8-menbered heterocycloalkenyl, C 3-8 cycloalkyloxy, C 3-8 cycloalkylamino, C 6 aryl, 5 or 6-membered heteroary, C 6 aryloxy, C 6 arylamino, C 6 arylcarbonyl or C 6 arylcarbonylamino each of which may have any group selected from substituent group G; a hydrogen atom; a halogen atom; a hydroxy group; amino; cyano; carboxy; C 1-6 alkyl, C 2-6 alkenyl, C 2-6 alkynyl, C 1-6 alkoxy, mono(di) C 1-6 alkylamino, C 2-7 acyl, C 2-7 acylamino, mono(di)C 1-6 alkylcarbamoyl, C 1-6 alkylsulfonyl, C 1-6 alkylsulfonylamino, mono(di)C 1-6 alkylsulfamoyl, C 1-6 alkylthio, C 3-8 cycloalkyl, 3 to 8-membered heterocycloalkyl, C 5-8 cycloalkenyl, 5 to 8-menbered heterocycloalkenyl, C 3-8 cycloalkyloxy, C 3-8 cycloalkylamino, C 6 aryl, 5 or 6-membered heteroary, C 6 aryloxy, C 6 arylamino, C 6 arylcarbonyl or C 6 arylcarbonylamino each of which may have any group selected from substituent group G; substituent group G consists of a fluorine atom, a chlorine atom, a hydroxy group, amino, carboxy, carbamoyl, cyano, C 1-6 alkyl, C 1-6 alkoxy and mono(di)C 1-6 alkylamino; ring U represents a benzene ring or 5 or 6-membered heteroaryl; m represents an integral number from 0 to 2; and R 2 represents a fluorine atom, a hydroxy group, amino, methyl or trifluoromethyl, and when m is 2, two R 2 are optionally different from each other, or a prodrug thereof, or a pharmaceutically acceptable salt thereof, in which "prodrug" means a compound wherein any one or more groups selected from a hydroxyl and a carboxy groups of a compound represented by the above formula is substituted by a group forming a prodrug, in which the group forming a prodrug is selected from the group consisting of: (in a hydroxyl group), C 1-6 alkyl-CO-, C 6-10 aryl-CO-, C 1-6 alkyl-O-C 1-6 alkylene-CO-, C 1-6 alkyl-OCO-C 1-6 alkylene-CO-, C 1-6 alkyl-OCO-, C 1-6 alkyl-O-C 1-6 alkylene-OCO-, C 1-6 alkyl-COO-C 1-6 alkylene, C 1-6 alkyl-OCOO-C 1-6 alkylene, C 3-8 cycloalkyl-OCOO-C 1-6 alkylene, and an ester or an amide with an amino acid; (in a carboxy group), C 1-6 alkyl, C 1-6 alkyl-COO-C 1-6 alkylene, C 1-6 alkyl-OCOO-C 1-6 alkylene, and C 3-8 cycloalkyl-OCOO-C 1-6 alkylene. (in a hydroxyl group), C 1-6 alkyl-CO-, C 6-10 aryl-CO-, C 1-6 alkyl-O-C 1-6 alkylene-CO-, C 1-6 alkyl-OCO-C 1-6 alkylene-CO-, C 1-6 alkyl-OCO-, C 1-6 alkyl-O-C 1-6 alkylene-OCO-, C 1-6 alkyl-COO-C 1-6 alkylene, C 1-6 alkyl-OCOO-C 1-6 alkylene, C 3-8 cycloalkyl-OCOO-C 1-6 alkylene, and an ester or an amide with an amino acid; (in a carboxy group), C 1-6 alkyl, C 1-6 alkyl-COO-C 1-6 alkylene, C 1-6 alkyl-OCOO-C 1-6 alkylene, and C 3-8 cycloalkyl-OCOO-C 1-6 alkylene.