The purpose of the present invention is to provide, as a nucleic acid derivative having an immunostimulatory activity, a double-stranded oligonucleotide containing a CpG oligonucleotide as mentioned below. An adjuvant comprising a double-stranded oligonucleotide in which a first strand is a CpG oligonucleotide composed of 8 to 50 nucleotides and a second strand is an oligonucleotide composed of 8 to 60 nucleotides and containing a sequence capable of hybridizing with the first strand, wherein a lipid is bonded to the second strand with a linker interposed therebetween.