Abstract: [Problems] A highly sensitive and accurate determination of a test substance is achieved without the need of complicated procedures. [Means for solving the Problems] The presence or absence or the concentration of a test substance 3 can be determined by gathering around the surface of a working electrode 1 metal microparticles 5 in an amount corresponding to the amount of the test substance 3 contained in a test solution, oxidizing the metal microparticles 5 electrochemically, measuring the value of a current produced by electrochemically reducing the oxidized metal, and determining the presence or absence or the concentration of the substance 3 based on the current value. It is preferred that the electrochemical oxidation of the metal microparticles 5 be conducted while maintaining the potential of the working electrode 1 at a value equal to a potential employed for the electrochemical oxidization of the metal microparticles 5.