PROBLEM TO BE SOLVED: To provide a means for efficiently introducing a nucleic acid into cells.SOLUTION: A promoter for introducing the nucleic acid into cells which contains a collagen-like peptide treated by irradiating radiation, a nucleic acid/collagen-like peptide complex comprising the collagen-like peptide treated by the irradiation of radiation and the nucleic acid, and a pharmaceutical composition comprising the collagen-like peptide treated by the irradiation of radiation and the nucleic acid are provided.