ends right end of two lead wires 10 coated with fluorocarbon resin tubes 9 as shown in fig2 in this embodiment platinum electrodes each having a diameter of about 0 5 mm are used as the electrode portions 11 so that even if the probe electrode 8 is exposed to the plasma gas the electrodes 11 are not oxidized further an insulating tube 12 of aluminum oxide is fitted on each of the electrodes 11 so as to expose a top end portion about 10 mm in this case of the electrodes 11 base end portions of the insulating tubes 12 are fixed through an epoxy adhesive 13 so that the distance between the electrodes 11 is held to be a predetermined value about 2 mm in this case further a fluorocarbon resin sealing tape 14 is wound around the exterior of the epoxy adhesive 13 so as to eliminate any influence to the plasma by gas generated from the epoxy adhesive 13 a dc power supply 15 is connected to the input terminal of the probe electrode 8 as shown in fig1 in this embodiment a constant voltage regulated power supply gp060 3 produced by takasago is used as the dc power supply 15 and is set so as always to apply a voltage of 30 v across the electrodes 11 of the probe electrode 8 when such voltage is applied a probe current flows across the electrodes 11 the probe current varies in proportion to the ion density in a plasma atmosphere a resistor 19 is connected between the output terminal of the probe electrode 8 and the dc power supply 15 the resistor 19 serves.