An antisense oligonucleotide according to claim 7, wherein B is a group selected from the group consisting of a 6-benzoylamino-9-yl group, a 2-isobutyrylamino-6-hydroxypurin group, a uracinyl group, a 2-oxo-4-hydroxy-5-methylpyrimidin-1-yl group, a 2-oxo-4-benzoylamino-pyrimidin-1-yl group and a 2-oxo-5-methyl-4-benzoylamino-pyrimidin-1-yl group.