To provide a gene transfer agent in which adsorption of serum protein is inhibited.

The gene transfer agent has a branch type polymer containing an aromatic ring as a nuclei and having a plurality of cationic branched chains extending radially therefrom and the gene transfer agent is composed of a crosslinked polymer in which a plurality of the cationic branched polymers are crosslinked to each other through a nonionic crosslinked chain. The nonionic crosslinked chain may be a branched polymer having a nonionic branched chain. The gene transfer agent can condense a nucleic acid such as DNA in high density by advantage on the structure. Since the gene transfer agent has a plurality of cationic branched polymers which are crosslinked, the gene transfer agent includes nucleic acids such as DNA in wider network, compared with a gene transfer agent composed of one cationic branched polymer and exhibits excellent gene transfer activity.