A 1,2,4-triazolone derivative represented by Formula: in Formula, R 1 represents a C 1-5 alkyl optionally substituted by one to three groups selected from the group consisting of hydroxy, halogen atoms, cyano, C 3-7 cycloalkyl, and C 1- 5 alkoxy; C 3-7 cycloalkyl, or 4- to 8-membered saturated heterocycle; R 2 represents a hydrogen atom or C 1-5 alkyl; R 3 represents aryl or heteroaryl optionally substituted by one or two groups selected from the group consisting of C 1-5 alkoxy, C 1-5 alkyl, halogen atoms, trifluoromethyl, trifluoromethoxy, cyano, hydroxy, difluoromethoxy, and C 1-5 alkylsulfonyl; R 4 and R 5 may be the same or different and each represent a hydrogen atom, C 1-5 alkyl optionally substituted by one to three groups selected from the group consisting of hydroxy, halogen atoms, cyano, C 3-7 cycloalkyl, and C 1-5 alkoxy; C 3-7 cycloalkyl, or 4- to 8-membered saturated or unsaturated heterocycle containing one or more nitrogen, oxygen, or sulfur atoms in the ring optionally substituted by one or two groups selected from the group consisting of hydroxy, C 1-5 alkyl, C 1-5 alkoxy, halogen atoms, cyano, C 2-5 alkanoyl, and trifluoromethyl; or R 4 and R 5 optionally, together with the adjoining nitrogen atom, form a 4- to 8-membered saturated or unsaturated heterocycle optionally containing one or more nitrogen, oxygen, or sulfur atoms, in addition to the adjoining nitrogen atom, in the ring optionally substituted by one or two groups selected from the group consisting of hydroxy, C 1-5 alkyl optionally substituted by one or two hydroxy; C 1-5 alkoxy, halogen atoms, cyano, C 2-5 alkanoyl, oxo, aminocarbonyl, mono-C 1-5 alkylaminocarbonyl, di-C 1-5 alkylaminocarbonyl, trifluoromethyl, and amino optionally substituted by one or two groups selected from the group consisting of C 1-5 alkyl and C 2-5 alkanoyl; and the 4- to 8-membered saturated or unsaturated heterocycle optionally has a C 1-5 alkylene group crosslinking two different carbon atoms in the ring or form 2-oxa-6-azaspiro[3.3]hept-6-yl or 7-oxa-2-azaspiro[3.5]non-2-yl; A represents phenylene or 6-membered heteroarylene optionally substituted by one or two groups selected from halogen atoms and C 1-5 alkoxy; X represents a single bond, -O-, or -NR 6 -; R 6 represents a hydrogen atom, C 1-5 alkyl, or C 2-5 alkanoyl; R a represents a hydrogen atom or C 1-5 alkyl; and n is an integer of 0 to 3, or a pharmaceutically acceptable salt of the 1,2,4-triazolone derivative. R 1 represents a C 1-5 alkyl optionally substituted by one to three groups selected from the group consisting of hydroxy, halogen atoms, cyano, C 3-7 cycloalkyl, and C 1- 5 alkoxy; C 3-7 cycloalkyl, or 4- to 8-membered saturated heterocycle; R 2 represents a hydrogen atom or C 1-5 alkyl; R 3 represents aryl or heteroaryl optionally substituted by one or two groups selected from the group consisting of C 1-5 alkoxy, C 1-5 alkyl, halogen atoms, trifluoromethyl, trifluoromethoxy, cyano, hydroxy, difluoromethoxy, and C 1-5 alkylsulfonyl; R 4 and R 5 may be the same or different and each represent a hydrogen atom, C 1-5 alkyl optionally substituted by one to three groups selected from the group consisting of hydroxy, halogen atoms, cyano, C 3-7 cycloalkyl, and C 1-5 alkoxy; C 3-7 cycloalkyl, or 4- to 8-membered saturated or unsaturated heterocycle containing one or more nitrogen, oxygen, or sulfur atoms in the ring optionally substituted by one or two groups selected from the group consisting of hydroxy, C 1-5 alkyl, C 1-5 alkoxy, halogen atoms, cyano, C 2-5 alkanoyl, and trifluoromethyl; or R 4 and R 5 optionally, together with the adjoining nitrogen atom, form a 4- to 8-membered saturated or unsaturated heterocycle optionally containing one or more nitrogen, oxygen, or sulfur atoms, in addition to the adjoining nitrogen atom, in the ring optionally substituted by one or two groups selected from the group consisting of hydroxy, C 1-5 alkyl optionally substituted by one or two hydroxy; C 1-5 alkoxy, halogen atoms, cyano, C 2-5 alkanoyl, oxo, aminocarbonyl, mono-C 1-5 alkylaminocarbonyl, di-C 1-5 alkylaminocarbonyl, trifluoromethyl, and amino optionally substituted by one or two groups selected from the group consisting of C 1-5 alkyl and C 2-5 alkanoyl; and the 4- to 8-membered saturated or unsaturated heterocycle optionally has a C 1-5 alkylene group crosslinking two different carbon atoms in the ring or form 2-oxa-6-azaspiro[3.3]hept-6-yl or 7-oxa-2-azaspiro[3.5]non-2-yl; A represents phenylene or 6-membered heteroarylene optionally substituted by one or two groups selected from halogen atoms and C 1-5 alkoxy; X represents a single bond, -O-, or -NR 6 -; R 6 represents a hydrogen atom, C 1-5 alkyl, or C 2-5 alkanoyl; R a represents a hydrogen atom or C 1-5 alkyl; and n is an integer of 0 to 3,