PURPOSE: To use an optical fiber of a non-conductor to measure safely and surely the DC electric field in an optional position, by screening periodically the DC electric field applied to parallel conductor plates and short-circuiting the electric charge between parallel conductor plates when it is screened.

CONSTITUTION: When a driving motor 14 is rotated, conductor screening plates 12 and 12a screen parallel conductor plates 9 and 9a of a BSO sensor I, and the electric charge stored in parallel conductor plates 9 and 9a is short-circuited through brushes 16 and 16a by a short-circuiting plate 13. When the motor 14 is rotated furthermore, screening plates 12 and 12a are removed, and a DC electric field acts upon parallel conductor plates 9 and 9a, and an alternating electric field is applied to the BSO sensor I. Meanwhile, the light incident through an optical fiber 6 is inputted to a photo detecting device through a lens 5, a polarizer 1, a 1/4 wavelength plate 2, a BSO single crystal 3, a photodetector 4, a lens 5a, and an optical fiber 6a, and the intensity of the electric field is measured on a basis of the variance of the quantity of light.