PURPOSE: To detect the presence of an immunoactive substance quantitatively by measuring variation of impedance caused by bringing an electrode having a bonding position specific to the immunoactive substance into contact with a medium having an enzyme substrate.

CONSTITUTION: An electrode is obtained by making a layer of immunoactive substance 22 adsorbed onto a thin metallic film 6 formed on a substrate 4. An electrode 2 is immersed into a medium being tested for the presence of immunoactive substance 24. When the substance 24 is present, it is bonded to a bonding position on the substance 22. The electrode 2 is then brought into contact with a medium containing an immunoactive substance 26 which is not bonded to the substance 24 being bonded to the substance 22. Finally, the electrode 2 is brought into contact with a medium containing an enzyme substrate for the enzyme bonded to the substance 26. When a molecular layer 30 is formed, the electrode is coated with the layer 30 and the impedance thereof is varied.