The purpose of the present invention is to provide double-stranded oligonucleotides comprising the CpG oligonucleotide mentioned below, as a nucleic acid derivative having an immunostimulatory activity. An adjuvant comprising a double-stranded oligonucleotide, wherein a first strand is a CpG oligonucleotide consisting of 8 to 50 nucleotides, a second strand is an oligonucleotide consisting of 8 to 60 nucleotides and comprising a sequence capable of hybridizing with the first strand, and a lipid binds to the second strand through a linker or without a linker.
