Disclosed is a new type of immunostimulating agent including an immunostimulating oligonucleotide complexed with a carrier which is safe and has a high transfection effect. The carrier complexed with the immunostimulating oligonucleotide to form the immunostimulating agent is a polysaccharide having β-1,3-bonds (preferably β-1,3-glucan such as schizophyllan). A preferred example of the immunostimulating oligonucleotide is one containing an unmethylated CpG motif. The polysaccharide for use is preferably modified with nucleic acid-binding functional group and/or cell membrane-affinitive functional group.