A single stranded nucleic acid probe having a base sequence complementary to the gene to be detected is immobilized onto the surface of an electrode or the tip of an optical fiber, and the nucleic probe is reacted with the gene sample denatured to a single stranded form, and then the nucleic acid probe hybridized with the gene is detected. In this procedure, to the reaction system consisting of the nucleic acid probe and the gene sample, a double stranded nucleic acid recognizing substance capable of binding specifically to the double stranded nucleic acid and being active electrochemically or optically is added. The detection of the nucleic acid probe is conducted by electrochemical or optical determination utilizing the electrode or optical fiber mentioned above. By this method, safer and more convenient detection of the gene is possible at a higher sensitivity even in a reduced time period.