PURPOSE: To prevent the effect due to disturbance noise or a leak current, by providing an guard electrode part with a coaxial structure composed of a center conductor, an insulating cylinder and an external electrode cylinder and forming an uneven ring on the outer peripheral surface of an insulating cylinder member.

CONSTITUTION: The guard electrode part 6 provided to the center of a detection part main body 1 is allowed to have a coaxial structure composed of a center conductor 7, an insulating cylinder 8 and an external electrode cylinder 9, and high voltage is applied to the conductor 7 and the electrode cylinder 9 from a circuit receiving part 4 through a resistor R to bring both of them to the same potential. Therefore even when said electrode part 6 is guarded by the electrode cylinder 9 in the same way as a usual coaxial cable and receives disturbance noise, there is no effect on anode voltage. Further, by forming the outer peripheral surface of the first insulating member 16 having the electrode part 6 fixed to the center thereof into a plurality of uneven ring shapes and making the creepage distance from the electrode part 6 to a cathode 22 sufficiently long, leakage at the time of high humidity can be suppressed to the min.