A radiation monitor comprising: - radiation detection means for detecting a γ ray emitted from N-16 nuclide which is a measurement target nuclide and outputting an analog voltage pulse; and - radiation measurement means for receiving the analog voltage pulse output from the radiation detection means, and measuring and outputting radiation in a measurement energy range, wherein the radiation measurement means includes - pulse amplification means arranged for amplifying the input analog voltage pulse and removing superimposed high frequency noise, - high-energy count-rate-measurement means (22a, 22b) arranged for discriminating the analog voltage pulse output from the pulse amplification means by a high-energy window and a low-energy window which are set so as not to be superimposed on each other in accordance with a voltage level, respectively, measuring and outputting a high-energy count rate by processing a time constant of the pulses entering the high-energy window so that a standard deviation of the pulses becomes constant and the time constant is in inverse proportion to the high-energy count rate, and outputting an alert, when the high-energy count rate is increased beyond an acceptable set value, - low-energy count-rate-measurement means arranged for measuring and outputting a low-energy count rate by performing a moving average operation of the pulse entering the low-energy window for a constant measurement time, - alert-diagnosis means for determining whether or not the low-energy count rate is in a set acceptable range, when an alert is output from the high-energy count-rate-measurement means (22a, 22b), determining that the alert is caused by fluctuation, when the low-energy count rate is in the set acceptable range, determining that the alert is caused by any one of an increase in the γ ray which is the measurement target or enter of noise, when the low-energy count rate increases beyond the set acceptable range, and outputting a result of the determination, and - display/user-operation means for displaying each output and performing operations and settings of each means. - radiation detection means for detecting a γ ray emitted from N-16 nuclide which is a measurement target nuclide and outputting an analog voltage pulse; and - radiation measurement means for receiving the analog voltage pulse output from the radiation detection means, and measuring and outputting radiation in a measurement energy range, - pulse amplification means arranged for amplifying the input analog voltage pulse and removing superimposed high frequency noise, - high-energy count-rate-measurement means (22a, 22b) arranged for discriminating the analog voltage pulse output from the pulse amplification means by a high-energy window and a low-energy window which are set so as not to be superimposed on each other in accordance with a voltage level, respectively, measuring and outputting a high-energy count rate by processing a time constant of the pulses entering the high-energy window so that a standard deviation of the pulses becomes constant and the time constant is in inverse proportion to the high-energy count rate, and outputting an alert, when the high-energy count rate is increased beyond an acceptable set value, - low-energy count-rate-measurement means arranged for measuring and outputting a low-energy count rate by performing a moving average operation of the pulse entering the low-energy window for a constant measurement time, - alert-diagnosis means for determining whether or not the low-energy count rate is in a set acceptable range, when an alert is output from the high-energy count-rate-measurement means (22a, 22b), determining that the alert is caused by fluctuation, when the low-energy count rate is in the set acceptable range, determining that the alert is caused by any one of an increase in the γ ray which is the measurement target or enter of noise, when the low-energy count rate increases beyond the set acceptable range, and outputting a result of the determination, and - display/user-operation means for displaying each output and performing operations and settings of each means.