A detecting unit (2) detects radiation. A data acquiring block (34) creates basic data equivalent to the count per unit time of the detected radiation. A data processing block (40) compares the basic data with a threshold and generates a annunciation control signal when the basic data is above the threshold. An annunciation block (42) generates a detection sound in response to the annunciation control signal. A computing block (38) computes the threshold by using a coefficient specified by means of an input device (18) and the maximum value of the basic data stored in a storage block (36). When the user repetitively examines a region (50) to be measured while gradually increasing the threshold, the region where the detection sound is generated is gradually narrowed. With this, a portion (53) where radiation is concentrated in the region (50) can be found out.