The present invention provides a vector which can make nucleic acids to be an aggregate and locate a vector containing a cationic polymer to surround the aggregate so as to protect the nucleic acids from the enzyme. The vector comprises a polymer having branched chain(s). Preferably, 3, 4, or 6 branched chains are bonded to benzene ring. It is preferable that the number of the branched chains is higher. As the branched chain(s), a vinyl-series acrylic polymer is preferable. The vector is synthesized by reacting a dithiocarbamate compound with an acrylamide monomer for the branched chain(s).
