A nucleic acid-containing complex, containing a nucleic acid and a biodegradable polymer, especially a positively-charged water-insoluble biodegradable polymer, is disclosed. The complex has excellent properties of sustainedly releasing a desired nucleic acid, especially DNA, to a site in need of a treatment. Since the complex can be taken up to phagocytes such as macrophages and delivered specifically to the target site, the function of the nucleic acid can be exhibited in a target site specific manner, and thus more specific gene therapy can be achieved. The complex has no adverse effects, such as occurrence of recombinants or toxicity which could be caused by using a virus vector such as adenovirus, or liposome. Thus, the complex is particularly preferable for the field of gene therapy. Furthermore, the complex enhances the biological effect of the nucleic acid introduced into the cells, allowing a gene therapy with a lower dose of nucleic acids.