A radiation detector outputs an analog pulse for incident radiation, and a signal processing portion is furnished with a wave height measuring function of converting the analog pulse inputted therein to a digital form and then measuring a peak wave height of the analog pulse and a wave height spectrum measuring function of measuring a wave height spectrum on the basis of measured wave height data, computes a dose rate and mean energy on the basis of measured wave height spectral data, and outputs computation results. The signal processing portion computes the dose rate and the mean energy on the basis of the wave height spectral data in a same wave height range on a same time axis. It thus becomes possible to provide accurate information based on which to determine whether a rise in dose rate is contributed by natural radon and thoron or contributed by a reactor facility.