Measuring device and analytical element

The production amount of thiol compounds which is the product of the cyclic reaction of enzyme-labeled antibody, or the production rate thereof is measured as an adsorption rate on a gold electrode formed on an insulated gate field-effect transistor. The adsorption rate is measured by monitoring in real time the change in a potential on the gold electrode associated with the formation of a self-assembled monolayer on the gold electrode, that is, the current between a source and a drain in the insulated gate field-effect transistor. The measured adsorption rate is recorded by using a signal processing circuit and a data processing unit. Then, the amount of antigen is found from the adsorption rate. During this measuring, a high frequency voltage is applied to a reference electrode from a power supply to reduce the effect of external variations of the measurement.