PURPOSE: To enable the voltage of an object to be measured two-dimensionally measured with a higher spatial resolution and a wider area by a method wherein an electrooptical material is arranged at each part corresponding to cores of an optical fiber plate and an EO probe is formed corresponding to each of the cores equivalently.

CONSTITUTION: An EO probe 320 uses a fiber plate while an electooptical material 122 is incorporated into a core 126 at the tip part of the plate. The optical material 122 is arranged in the core 126 only and separated spatially. Incident light and the reflected light are both propagated through the core 126 and an optical path is also separated. As a result, the light is modulated only with the electrooptical material 122 at the tip of the core 126 and mixing with the light modulated is checked with the other optical material 122. An image is generated on a B surface of the top surface of the plate by the individual light thus modulated to be formed on a two-dimensional photo-detector 160. Light modulation thus caused with the individual optical material 122 is detected with the detector 160 and the two-dimensional voltage distribution of a device to be measured can be measured.