PURPOSE: To judge the abnormality of a dose monitor regardless of the value of transmitting radioactive rays by making ionization spaces formed between electrodes equal, and feeding the reverse-polarity voltage to one of a pair of electrodes of the dose monitor.

CONSTITUTION: The ionization current Ia of an ionization space A flows as shown in the Figure by radioactive rays. When the output of a bipolar high- voltage power source 6 is the same as that of a high-voltage power source 4, the ionization current Ia of the ionization space A and the ionization current Ib of an ionization space B are the same value in the same direction. The measured circuit current is 21a (21b). When the output of the bipolar high- voltage power source 6 has the reverse polarity, Ib and Ia are the same value in opposite directions. The measured circuit current is 0. When the voltage is deformed, the ionization currents Ia, Ib are not equal, and the measured circuit current is not 0. The abnormality of a dose monitor can be judged regardless of the value of transmitting radioactive rays.