An object of the present invention is to achieve a method capable of increasing transfection efficiency of polyethyleneimine (PEI) and reducing toxicity thereof, to provide a PEI with high transfection efficiency, and to provide a method of introducing nucleic acids into cells comprising using the PEI.There are provided a method of introducing nucleic acids into cells comprising mixing acidified PEI with nucleic acids under acidic conditions and applying the resulting mixture to cells, a reagent for introducing nucleic acids into cells comprising an acidified PEI solution, and a reagent kit including the solution.