There is provided a voltage detecting device using an electro-optical material which has its refractive index changed by a voltage provided on an object under test. The voltage detecting device consists of a strip line formed on the upper surface of the electro-optical material and an electrode formed on the bottom surface of the electro-optical material. One or both of the strip line and electrode may be formed of transparent conductive materials. In operation, a light beam is passed either from the top or the bottom of the device through the electro-optical material and the change in the polarization of the light is measured. That change is related to the voltage level of the voltage signal passing through the strip line formed on the material.