Problem The purpose of the present invention is to provide: a stereo isomer of a novel CpG oligonucleotide, which has excellent stability； and a CpG oligonucleotide which has a capability of producing interferon-± (IFN±). Solution The present invention relates to an oligonucleotide which contains two to four sequences each represented by the formula 5'-X 1 X 2 CpGX 3 X 4 -3' (formula (I)) and has a length of 14 to 32 nucleotides. In formula (I), CpG represents a non-methylated CpG residue having a phosphate skeleton modification, X 1 X 2 represents any one of AA, AT, GA and GT, and X 3 X 4 represents any one of TT, AT, AC and CG. The oligonucleotide has at least one phosphate skeleton modification at an S-form stereoisomer located at a site other than the CpG.
