The electric potential measuring instrument comprises an electrode arranged on a semiconductor substrate at a position opposite to the object of measurement and a modulator for modulating the coupling capacitance between the object of measurement and the electrode. The electrode is the gate electrode of a field effect type transistor. The modulated electric current that flows between the source diffusion region and the drain diffusion region of the field effect type transistor is synchronously detected by a detection circuit with the modulation frequency of the modulator. The output signal that appears on the gate electrode of a field effect transistor can be measured with ease by changing the coupling capacitance between the electrode of an electric potential measuring instrument and an object of measurement.