Provided is an immunostimulating oligonucleotide that is suitable for industrial production and has an excellent type-I IFN inducing activity even when not modified to become a phosphorothioate. This linear double-stranded oligonucleotide contains 10-100 base pairs, wherein the single-stranded oligonucleotides constituting the double-stranded oligonucleotide each contain 2-20 phosphodiester-mediated cytosine-guanine (CpG) sequences, and at least 90% of the internucleotide bonds in each single-stranded oligonucleotide are phosphodiester bonds.
