PURPOSE: To enable the measurement of a conductor voltage of a closed equipment safely and easily by transmitting voltage induced in a metal flat plate facing a conductor in a closed case once converted into light.

CONSTITUTION: A metal flat plate 11 is provided facing a bus conductor 1 sealed into a bus case 3 together with an insulating gas. The flat plate 11 is connected to a divider comprising metal flat plates 5W7 resin molded through a metal pipe 8. The flat plate 7 is connected to the flat plate 6 while led out through a metal rod 10 piercing a metal pipe 9 earthed and then, connected to a optical electric field sensor 21. Light from a light emitting element 25 is converted with the sensor 21 into a light with the intensity corresponding to voltage of the conductor 1 and then, sent to a light receiving element 26. Thus, the voltage of the conductor 1 is indicated on a display 27.