A multi-channel voltage detector for converting multi-channel electrical signals into optical signals and simultaneously detecting multi-channel voltages in parallel mode. Comprising electro-optic conversion means provided thereon with a plurality of electrical circuits for receiving corresponding multi-channel electrical signals to change the refractive index at each of the locations thereon according to the supplied multi-channel electrical signals, means for supplying light having a predetermined polarization to each of the locations, each of the emergent light beams transmitted through the conversion means having a polarization different from said predetermined polarization in accordance with the change in the refractive index of the corresponding location, and photodetecting means for converting the changes in polarization of the emergent light beams to the changes in optical intensity thereof and for simultaneously detecting the changes in optical intensity of the emergent light beams in parallel to thereby simultaneously detect the multi-channel electrical signals in parallel.