Disclosed is a biosensor for electrochemically detecting the presence of an analyte and measuring its concentration. In the biosensor, an organic substance as a substrate is reduced to a substance having an amine group by the catalysis of a nanocatalyst label with a plurality of active sites in the presence of a reducing agent, the reduction is continuously carried out to amplify the substance having an amine group, and the amount of the substance having an amine group is measured by taking advantage of the fact that the substance having an amine group exhibits different electrochemical, chromatic or fluorescent properties from those of the substrate, so that the concentration of a biomarker is measured with high sensitivity within a short time. The electrochemical biosensor allows simple measurement of an analyte and has a low detection limit.