A novel transversal electro-optical switch using Bi12SiO20 single crystal is proposed and demonstrated. Since the drive voltage of switches of this type is proportional to the crystal thickness, we used a crystal plate with a thickness of only 100 µm to reduce drive voltage. With plate of this thickness, however, the light rays propagated inside the crystal undergo total internal reflection at its upper and lower surfaces. Consequently, the polarization state is scrambled causing crosstalk. A thin three layer interference film of SiO2-Si:H-SiO2 was applied to the surfaces of the crystal to maintain the polarization state. By this method, a drive voltage of 390 V with crosstalk of -20 dB was obtained at 1.3 µm wavelength.