PURPOSE: To reduce the number of measurement cables, to facilitate wiring, and to reduce dispersion of measurement values by thinning a second conductor, and by practically eliminating change in the electrostatic capacity between a first conductor and a third conductor due to existence of the second conductor.

CONSTITUTION: Conductors 201, 303, 205, 207 are arranged concentrically through insulators 202, 204, 206, and when in use, the conductors 201 and 303 are of practically the same electric potential. The capacity Cg between the conductor 205 to be connected to a guard electrode G and the conductor 303 is determined according to an expression Cg=2π/log(R5/R3), wherein π is a ratio of the circumference of a circle to its diameter, and is the dielectric constant of an insulator, while R3 is the outer diameter of the conductor 303 and R5 is the inner diameter of the conductor 205. The conductor 303, when used, is used for detecting voltage, and since the inductance thereof does not largely affect a measurement system, the conductor 303 is made to be sufficiently thin, and is arranged in the vicinity of the conductor 201. The change in the conductive capacity between the conductors 201 and 205 is practically eliminated due to the existence of the conductor 303, and the number of measurement cables 300 is reduced, while wiring is facilitated and dispersion of measurement values is reduced.