Provided is an adjuvant composition comprising a nucleic acid comprising a double-stranded RNA bound to a single-stranded DNA, the double-stranded RNA consisting of a nucleotide sequence of SEQ ID NO: 1 and its complementary sequence, the single-stranded DNA consisting of a nucleotide sequence of SEQ ID NO: 2. Also provided is a vaccine composition comprising the adjuvant composition and an antigen or an antigen component. The nucleic acid contained in the adjuvant composition is a chemically synthesizable nucleic acid having potent adjuvant activity and high safety.
