PURPOSE: To widen a usable temp. range, by a method wherein two coaxial cables are attached and the central conductor of one cable is connected to an anode while the central conductor of the other one is connected to a cathode and voltage is applied to the cathode and the anode in an allotted state.

CONSTITUTION: When voltage required in operating a proportional counter is set to VO and voltage with +VO/2 and voltage of -VO/2 are supplied by two DC high voltage power sources 105a, 105b, because voltage VO is applied between the cathode 1 and the anode 2 of the proportional counter, this proportional counter is normally operated while an electric pulse signal generated in the proportional counter is amplified by an amplifier. When the amplified pulse signal is counted by a counting circuit, the counting of radioactive rays can be carried out. On the other hand, because voltages +VO/2 applied to coaxial cables 6a, 6b may be the half of the operation voltage VO of the proportional counter, the possibility of the generation of a minute discharge pulse in the cable becomes extremely small and said counter can be sufficiently used even at a temp. higher than a conventional limit environmental temp.