[en] To observe the airborne gamma radiation dose rate, monitoring posts are set up at the border of a supervised area of JNC-OEC. Measurement values of some ionization chambers set at monitoring posts were increased by signals of unknown origin occurring at random times. To probe the cause, measurement of electric field intensity at the ionization chamber and immunity test at anechoic chamber were carried out. Result of examination made clear that measurement values are increased by a specific frequency band electromagnetic wave. There were also clear indications that ferrite cores and shield tube are effective in eliminating electromagnetic wave noise. When attaching ferrite cores to cables of ionization chambers, unknown increase of measurement values can be stopped. (author)