Abstract: A biochemical reactant is detected by means of a biochip with good reproducibility. A nucleic acid probe is made to have a three-dimensional structure by adopting a loop structure in such a way that a free end not fixed to the surface of a substrate or a portion with a modifiable label is located on an electrode of a biochip or near the surface of the electrode on the substrate side. Alternatively, an essential portion complementarily connected to the biochemical specimen can be located on the substrate side. As a result the nucleic acid probe has a property that it can be basically hybridized with only the object specimen. When the nucleic acid probe is hybridized with the sample specimen, the loop structure is destroyed, and the label of only the hybridized probe can be modified.