PURPOSE: To make an electric field sensor compact, by coupling the polarized light from a first polarization-plane preserved fiber, having a light guide at the intermediate part so that the light guide is formed by coating the outer surface of optical fiber with an electrostriction material, and the polarized light from a second polarization-plane preserved fiber.

CONSTITUTION: Polarized light from a polarized-light emitting light source 5 is split into two luminous fluxes through a beam splitter 6. One luminous flux is optically coupled with the front end of a first polarization-plane preserved fiber 2. The other is optically coupled with the front end of a second polarization-plane preserved fiber 2'. When an electric field is applied on a light guide 3, strain is yielded in an electrostriction material 4. Strain is also yielded in the light guide 3. Then the phase of the light in the second polarization-plane preserved fiber 2 is changed. The outputs of the polarization- plane preserved fibers 2 and 2' are made to interfere in an optical coupler 7. Then the intensity of the electric field is detected by the fluctuation of the interference due to the intensity of the electric field, which is applied on the electrostriction material 4.