The present invention is to provide a method which makes it possible to activate RecA, and maintain its biological function in all the PCR cycles. Also, the invention is to provide a nucleic acid amplification method, and a kit for amplifying a nucleic acid which makes it possible to suppress non-specific amplification more specifically and efficiently to amplify only the desired nucleic acid. A method of activating a RecA protein derived from an extremely thermophilic bacterium in a polymerase chain reaction carried out in the presence of the RecA protein derived from the extremely thermophilic bacterium, wherein the RecA protein is activated by carrying out the reaction with the addition of nucleotide 5′-triphosphate (provided that the nucleotide 5′-triphosphate is neither deoxynucleotide 5′-triphosphate nor nucleotide 5′-O-3-thiotriphosphate).