A radiation measuring device includes a computing unit that receives an input of a count value of a count portion configured to receive an input of a digital pulse from a pulse height discriminator and count the digital pulse in a fixed cycle and an integrated value of an adder-subtractor accumulation portion configured to add the digital pulse and subtract a feedback pulse and finds a first current rate on the basis of the count value and the integrated value and a second count rate on the basis of the integrated value, so that soundness of the adder-subtractor accumulation portion is diagnosed and outputted by comparing the first count rate with the second count rate. A radiation measuring device of a simple configuration and a diagnosis method thereof enabling a high-accurate self-diagnosis online on the count rate measurement that is the keystone of the radiation measurement can be thus obtained.