PURPOSE: To improve the sensitivity by removing a noise current of low frequency characteristic to a semiconductor detector by a one-shot circuit and outputting only signal currents inputted from a couple of one-shot circuits at the same time by a simultaneous counting circuit.

CONSTITUTION: Signal pulse is and a noise current in are amplified by amplifiers 7 and 7' and outputted to one-shot circuits 10 and 10'. The one-shot circuits 10 and 10' operates only with a fast-rising signal (high frequency signal) and does not respond to the noise signal of low frequency, so the one-shot circuits 10 and 10' output the signal pulses Is and a noise N induced on a signal line, so that the noise signal in is removed. Then, the simultaneous counting circuit 11 judges the outputs of the one-shot circuits 10 and 10' to output only the signal pulses Is which are inputted at the same time to a counting circuit 9 and remove the induced noise N because it is not inputted at the same time. Thus, noises characteristics to the semiconductor detector can be removed, so even low-energy radiation is detected.