An electric potential measuring apparatus includes a detecting electrode,
a modulator for changing an electrostatic capacitance between a
measurement object and the detecting electrode by mechanical vibration, a
charge detector for detecting the amount of charges electrostatically
induced in the detecting electrode by the modulator, and a vibration
generating-detecting unit for selectively performing either a vibration
generating function of exciting the mechanical vibration, or a detecting
function of detecting the condition of the mechanical vibration.