ProblemThe 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-α (IFNα).SolutionThe present invention relates to an oligonucleotide which contains two to four sequences each represented by the formula 5′-X1X2CpGX3X4-3′ (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, X1X2 represents any one of AA, AT, GA and GT, and X3X4 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.