A polarizing optical system (15, 16) is disposed perpendicular to an optical axis of incoming light from a light source (12), having the optical axis (12) as a center axis, and configured for polarization of incoming light to a prescribed reference state, an electro-optical device (17) is disposed perpendicular to the optical axis, having the optical axis as a center axis, and adapted, as a voltage to be measured is imposed thereon, to respond to the imposed voltage by polarizing light polarized by the polarizing optical system (15, 16), and an analyzer (18) is disposed perpendicular to the optical axis, having the optical axis as a center axis, and adapted for detection of light polarized by the electro-optical device (17), to irradiate a detector (21) configured for conversion of incoming light into an electric signal.