A compound represented by formula I or II below or a salt thereof: (wherein B 1 represents a purine-9-yl group or a 2-oxo-1,2-dihydropyrimidine-1-yl group that may have any one or more substituents selected from a group α, wherein the group α consists of a hydroxyl group, a hydroxyl group protected by a protecting group in nucleic acid synthesis, a C 1 to C 6 linear alkyl group, a C 1 to C 6 linear alkoxy group, a mercapto group, a mercapto group protected by a protecting group in nucleic acid synthesis, a C 1 to C 6 linear alkylthio group, an amino group, a C 1 to C 6 linear alkylamino group, an amino group protected by a protecting group in nucleic acid synthesis, and a halogen atom; R 1, R 12, and R 13 each independently represent a hydrogen atom, a C 1 to C 7 alkyl group that may be branched or form a ring, a protecting group for an amino group, or and R 2 and R 3 each independently represent a hydrogen atom, a protecting group for a hydroxyl group in nucleic acid synthesis, a C 1 to C 7 alkyl group that may be branched or form a ring, a C 2 to C 7 alkenyl group that may be branched or form a ring, a C 3 to C 12 aryl group that may have any one or more substituents selected from the group α and that may have a hetero atom, an aralkyl group having a C 3 to C 12 aryl moiety that may have any one or more substituents selected from the group α and that may be a hetero atom, an acyl group that may have any one or more substituents selected from the group α, a silyl group that may have any one or more substituents selected from the group α, a phosphate group that may have any one or more substituents selected from the group α, a phosphate group protected by a protecting group in nucleic acid synthesis, or -P(R 4)R 5 (wherein R 4 and R 5 each independently represent a hydroxyl group, a hydroxyl group protected by a protecting group in nucleic acid synthesis, a mercapto group, a mercapto group protected by a protecting group in nucleic acid synthesis, an amino group, a C 1 to C 5 alkoxy group, a C 1 to C 5 alkylthio group, a C 1 to C 6 cyanoalkoxy group, and/or an amino group substituted with a C 1 to C 6 alkyl group)). (wherein B 1 represents a purine-9-yl group or a 2-oxo-1,2-dihydropyrimidine-1-yl group that may have any one or more substituents selected from a group α, wherein the group α consists of a hydroxyl group, a hydroxyl group protected by a protecting group in nucleic acid synthesis, a C 1 to C 6 linear alkyl group, a C 1 to C 6 linear alkoxy group, a mercapto group, a mercapto group protected by a protecting group in nucleic acid synthesis, a C 1 to C 6 linear alkylthio group, an amino group, a C 1 to C 6 linear alkylamino group, an amino group protected by a protecting group in nucleic acid synthesis, and a halogen atom; R 1, R 12, and R 13 each independently represent a hydrogen atom, a C 1 to C 7 alkyl group that may be branched or form a ring, a protecting group for an amino group, or and R 2 and R 3 each independently represent a hydrogen atom, a protecting group for a hydroxyl group in nucleic acid synthesis, a C 1 to C 7 alkyl group that may be branched or form a ring, a C 2 to C 7 alkenyl group that may be branched or form a ring, a C 3 to C 12 aryl group that may have any one or more substituents selected from the group α and that may have a hetero atom, an aralkyl group having a C 3 to C 12 aryl moiety that may have any one or more substituents selected from the group α and that may be a hetero atom, an acyl group that may have any one or more substituents selected from the group α, a silyl group that may have any one or more substituents selected from the group α, a phosphate group that may have any one or more substituents selected from the group α, a phosphate group protected by a protecting group in nucleic acid synthesis, or -P(R 4)R 5 (wherein R 4 and R 5 each independently represent a hydroxyl group, a hydroxyl group protected by a protecting group in nucleic acid synthesis, a mercapto group, a mercapto group protected by a protecting group in nucleic acid synthesis, an amino group, a C 1 to C 5 alkoxy group, a C 1 to C 5 alkylthio group, a C 1 to C 6 cyanoalkoxy group, and/or an amino group substituted with a C 1 to C 6 alkyl group)).