A compound of the formula or a salt thereof: wherein, X 1: C-R 11 or N; X 2: C-R 12 or N; R 11: i) H, ii) halogen, iii) -CN, or iv) -O-C 1-6 alkyl; R 12: H or halogen; R 1: i) H or ii) C 1-6 alkyl; R 2: i) C 1-6 alkyl which may be substituted with one or more substituent(s) selected from the group consisting of -OR 0 (s), halogen(s), -CONR 21 R 22 (s), phenyl(s) which may be substituted with one or more substituent(s) selected from the group consisting of halogen(s) and -COOR 0 (s), and heteroaryl(s) which is selected from the group consisting of pyrazolyl and triazolyl, ii) C 2-6 alkenyl, iii) C 2-6 alkynyl, iv) NR 23 R 24 or v) -COOR 0; R 21: C 1-6 alkyl; R 22: C 1-6 alkyl; R 23: C 1-6 alkyl; R 24: i) C 3-8 cycloalkyl, or ii) phenyl; or R 1, R 2, and a carbon atom bounded by R 1 and R 2 may interact to form a 4-tetrahydropyran ring so that the carbon atom bounded by R 1 and R 2 is a spiro atom; R 3, R 4: the same or different each other, i) C 1-3 alkyl which may be substituted with one or more substituent(s) selected from the group consisting of halogen(s) and -OH(s) or ii) C 2-6 alkenyl which may be substituted with one or more substituent(s) selected from the group consisting of -OH(s) and heteroaryl(s) which is selected from the group consisting of pyrazolyl and thienyl, wherein the heteroaryl may be substituted with one or more C 1-6 alkyl(s), or, R 3, R 4, and a carbon atom bounded by R 3 and R 4 may interact to form a 3-oxetane ring and the carbon atom bounded by R 3 and R 4 is a spiro atom; R 5: i) H, ii) C 1-6 alkyl which may be substituted with one or more -O-(C 1-6 alkyl)(s), iii) -O-C 1-6 alkyl, iv) halogen, v) -COO-(C 1-6 alkyl), or vi) C 3-8 cycloalkyl; R 6: i) H, ii) C 1-6 alkyl which may be substituted with one or more substituent(s) selected from the group consisting of -O-(C 1-6 alkyl(s) which may be substituted with one or more halogen(s)) and halogen(s), iii) -OH, iv) -O-(C 1-6 alkyl which may be substituted with one or more halogen(s)), v) halogen, vi) -CN, vii) -S-(C 1-6 alkyl), viii) C 3-8 cycloalkyl, ix)-NR 0 R 0, or x) C 2-6 alkenyl; R 0: the same or different each other, H or C 1-6 alkyl, provided that said compound is not methyl 1,1-diallyl-3-oxo-2,4-dihydroisoquinoline-4-carboxylate or a salt thereof. X 1: C-R 11 or N; X 2: C-R 12 or N; R 11: i) H, ii) halogen, iii) -CN, or iv) -O-C 1-6 alkyl; R 12: H or halogen; R 1: i) H or ii) C 1-6 alkyl; R 2: i) C 1-6 alkyl which may be substituted with one or more substituent(s) selected from the group consisting of -OR 0 (s), halogen(s), -CONR 21 R 22 (s), phenyl(s) which may be substituted with one or more substituent(s) selected from the group consisting of halogen(s) and -COOR 0 (s), and heteroaryl(s) which is selected from the group consisting of pyrazolyl and triazolyl, ii) C 2-6 alkenyl, iii) C 2-6 alkynyl, iv) NR 23 R 24 or v) -COOR 0; R 21: C 1-6 alkyl; R 22: C 1-6 alkyl; R 23: C 1-6 alkyl; R 24: i) C 3-8 cycloalkyl, or ii) phenyl; or R 1, R 2, and a carbon atom bounded by R 1 and R 2 may interact to form a 4-tetrahydropyran ring so that the carbon atom bounded by R 1 and R 2 is a spiro atom; R 3, R 4: the same or different each other, i) C 1-3 alkyl which may be substituted with one or more substituent(s) selected from the group consisting of halogen(s) and -OH(s) or ii) C 2-6 alkenyl which may be substituted with one or more substituent(s) selected from the group consisting of -OH(s) and heteroaryl(s) which is selected from the group consisting of pyrazolyl and thienyl, wherein the heteroaryl may be substituted with one or more C 1-6 alkyl(s), or, R 3, R 4, and a carbon atom bounded by R 3 and R 4 may interact to form a 3-oxetane ring and the carbon atom bounded by R 3 and R 4 is a spiro atom; R 5: i) H, ii) C 1-6 alkyl which may be substituted with one or more -O-(C 1-6 alkyl)(s), iii) -O-C 1-6 alkyl, iv) halogen, v) -COO-(C 1-6 alkyl), or vi) C 3-8 cycloalkyl; R 6: i) H, ii) C 1-6 alkyl which may be substituted with one or more substituent(s) selected from the group consisting of -O-(C 1-6 alkyl(s) which may be substituted with one or more halogen(s)) and halogen(s), iii) -OH, iv) -O-(C 1-6 alkyl which may be substituted with one or more halogen(s)), v) halogen, vi) -CN, vii) -S-(C 1-6 alkyl), viii) C 3-8 cycloalkyl, ix)-NR 0 R 0, or x) C 2-6 alkenyl; R 0: the same or different each other, H or C 1-6 alkyl, provided that said compound is not methyl 1,1-diallyl-3-oxo-2,4-dihydroisoquinoline-4-carboxylate or a salt thereof.