PURPOSE: To have easy judgement whether the characteristics of a sensor concerned have deteriorated, by furnishing a high tension electrode with a radiation source having such an intensity as capable of flowing a current with a current density equivalent to the measuring upper limit current density of the sensor.

CONSTITUTION: A high tension electrode 1 is equipped with a live zero rays source 22 having such an intensity as capable of flowing a current with a current density equivalent to the measuring upper limit current density of a sensor concerned. Electric dissociation current due to this live zero rays source 22 appears in the form of a change in the live zero amperage and a change in the saturation characteristics in the event of deterioration of the sensor characteristics. The saturation characteristics of the sensor are proportional to the square root of the saturation current density flowing when external radiations are incident to the sensor, and the saturation current density of the live zero current is equal to the saturation current density, i.e., the measuring upper limit value of the sensor, so that it becomes practicable to check the equivalent saturation characteristics data. Thus it can be judged easily whether the characteristics of the sensor have deteriorated.