<p>Disclosed is a method for determining the transcriptional regulation activity of a substance more accurately. A reporter gene assay method comprising: contacting a cell which carries a vector having a reporter gene ligated downstream to a transcription factor-recognition sequence and a nucleotide sequence needed for the initiation of transcription, with a substance to be tested and two types of detection antibodies, wherein the reporter gene has a gene encoding an epitope tag; and then detecting a phenomenon caused by the approach of the detection antibodies each other to establish the correlation between the phenomenon and the action of the substance on the transcriptional regulation mechanism.</p>
