A radiation detector comprising: a detecting portion that detects radiation to generate a detection signal; a data obtaining portion that generates basic data equivalent to a radiation count value per a predetermined unit time based on the detection signal; a first input device used for inputting a start signal that instructs on starting storing of a maximum value of the basic data; characterised by: a storing portion that receives said basic data from the data obtaining portion and stores only the maximum value of those basic data in response to the start signal; a second input device used for inputting information to designate a threshold to the basic data; a data processing portion that compares the basic data with the threshold and generates a notification control signal if the basic data exceeds the threshold; a notifying portion that executes notification in response to the notification control signal; and a threshold operation portion that calculates the threshold by use of the maximum value stored in the storing portion, wherein the threshold operation portion calculates the threshold according to the following expression: Ct = n × Cm wherein, Ct denotes the threshold, Cm denotes the maximum value, and a coefficient n satisfies 0<n 1, and as the information to designate the threshold, information to designate the coefficient n is inputted by use of the second input device. a detecting portion that detects radiation to generate a detection signal; a data obtaining portion that generates basic data equivalent to a radiation count value per a predetermined unit time based on the detection signal; a first input device used for inputting a start signal that instructs on starting storing of a maximum value of the basic data; characterised by: a storing portion that receives said basic data from the data obtaining portion and stores only the maximum value of those basic data in response to the start signal; a second input device used for inputting information to designate a threshold to the basic data; a data processing portion that compares the basic data with the threshold and generates a notification control signal if the basic data exceeds the threshold; a notifying portion that executes notification in response to the notification control signal; and a threshold operation portion that calculates the threshold by use of the maximum value stored in the storing portion, wherein the threshold operation portion calculates the threshold according to the following expression: Ct = n × Cm wherein, Ct denotes the threshold, Cm denotes the maximum value, and a coefficient n satisfies 0<n 1, and as the information to designate the threshold, information to designate the coefficient n is inputted by use of the second input device. a storing portion that receives said basic data from the data obtaining portion and stores only the maximum value of those basic data in response to the start signal; a second input device used for inputting information to designate a threshold to the basic data; a data processing portion that compares the basic data with the threshold and generates a notification control signal if the basic data exceeds the threshold; a notifying portion that executes notification in response to the notification control signal; and a threshold operation portion that calculates the threshold by use of the maximum value stored in the storing portion, wherein the threshold operation portion calculates the threshold according to the following expression: Ct = n × Cm wherein, Ct denotes the threshold, Cm denotes the maximum value, and a coefficient n satisfies 0<n 1, and as the information to designate the threshold, information to designate the coefficient n is inputted by use of the second input device.