For Faraday sensor element of the optical CT, a circular integral type made of flint glass was use to prevent influences claused by magnetic fields of other phases. For light transmission between the GIS tank and the sensor element of the GIS-type optical CT, a spatial transmission system in SF6 gas was adopted to ensure reliability of the electrical insulation for signal detection, a light power phase detection systems was employed because it permits easy control of errors and noise resulting from luminous energy variations and also permits a wide dynamic range to be used.