A compound of the formula (I) or a salt thereof: (In the formula, R 1 is H, C 1-6 alkyl which may be substituted with OH, C 3-8 cycloalkyl which may be substituted with R 00 , heterocycloalkyl which may be substituted with R 00 , phenyl which may be substituted with R 00 , heteroaryl which may be substituted with R 00 , -CO-C 1-6 alkyl, or -CO-C 3-8 cycloalkyl, in which R 00 represents substituents selected from the group consisting of C 1-6 alkyl, halogeno-C 1-6 alkyl, and halogen, R 2a is C 1-6 alkyl which may be substituted with R 01 , in which R 01 represents substituents selected from the group consisting of C 3-8 cycloalkyl, -O-(C 1-6 alkyl), -N(C 1-6 alkyl) 2 , -NH(C 1-6 alkyl), and -NH 2 , R 2b is H or C 1-6 alkyl, R 2a and R 2b may be combined with the same carbon atom in the piperazine ring to form C 3-8 cycloalkyl, R 3 is H or C 1-6 alkyl, R 4 is H or C 1-6 alkyl, X is *-CR 7 =CR 8 -, *-CR 7 =N-, *-N=CR 8 -, or S, in which * represents a bond with a carbon atom substituted with R 6 , R 5 , R 6 , and R 7 are the same as or different from each other, and are H, C 1-6 alkyl, -O-(C 1-6 alkyl), halogen, or CN, R 5 and R 6 may be combined with each other to form C 5-7 cycloalkenyl, R 8 is H or F, and the ring A is aryl which may be substituted with R 02 , C 5-7 cycloalkenyl-fused phenyl which may be substituted with R 02 , heteroaryl which may be substituted with R 02 , or C 6-8 cycloalkyl which may be substituted with R 02 , in which R 02 represents substituents selected from the group consisting of C 1-6 alkyl, halogeno-C 1-6 alkyl, C 3-8 cycloalkyl, -O-(C 1-6 alkyl), -O-(halogeno-C 1-6 alkyl), halogen, and -CN.) R 1 is H, C 1-6 alkyl which may be substituted with OH, C 3-8 cycloalkyl which may be substituted with R 00 , heterocycloalkyl which may be substituted with R 00 , phenyl which may be substituted with R 00 , heteroaryl which may be substituted with R 00 , -CO-C 1-6 alkyl, or -CO-C 3-8 cycloalkyl, in which R 00 represents substituents selected from the group consisting of C 1-6 alkyl, halogeno-C 1-6 alkyl, and halogen, R 2a is C 1-6 alkyl which may be substituted with R 01 , in which R 01 represents substituents selected from the group consisting of C 3-8 cycloalkyl, -O-(C 1-6 alkyl), -N(C 1-6 alkyl) 2 , -NH(C 1-6 alkyl), and -NH 2 , R 2b is H or C 1-6 alkyl, R 2a and R 2b may be combined with the same carbon atom in the piperazine ring to form C 3-8 cycloalkyl, R 3 is H or C 1-6 alkyl, R 4 is H or C 1-6 alkyl, X is *-CR 7 =CR 8 -, *-CR 7 =N-, *-N=CR 8 -, or S, in which * represents a bond with a carbon atom substituted with R 6 , R 5 , R 6 , and R 7 are the same as or different from each other, and are H, C 1-6 alkyl, -O-(C 1-6 alkyl), halogen, or CN, R 5 and R 6 may be combined with each other to form C 5-7 cycloalkenyl, R 8 is H or F, and the ring A is aryl which may be substituted with R 02 , C 5-7 cycloalkenyl-fused phenyl which may be substituted with R 02 , heteroaryl which may be substituted with R 02 , or C 6-8 cycloalkyl which may be substituted with R 02 , in which R 02 represents substituents selected from the group consisting of C 1-6 alkyl, halogeno-C 1-6 alkyl, C 3-8 cycloalkyl, -O-(C 1-6 alkyl), -O-(halogeno-C 1-6 alkyl), halogen, and -CN.) <EOS>