PURPOSE: To make easy the discrimination of types of incident radiation, by discriminating the detection pulse for the detected waveform of radiation from the difference of leading time.

CONSTITUTION: The α, β rays detected from each scintillator of the radiation detector 10 are amplified 12 and are fed to the differentiation circuits 14, 16. The output of the circuit 14 is fed to the peak comparison circuit 18, the output of the circuit 16 is amplified 20 and is given for the specified delay time at the delay 22 and fed to the circuit 18. The output of the circuit 14 and the output of the circuit 16 amplified are respectively fed to the output control circuits 28, 30 via the peak value discrimination circuits 24, 26 respectively. The circuits 28, 30 consist of the gate circuit and the output of the circuit 18 is fed to each gate input. The output of the circuits 28, 30 is fed respectively to the counter circuits 32, 34, and α ray is counted from the circuit 32 and β ray is detected from the circuit 34. Since the time constant of the circuits 14, 16 performing wave peak discrimination is set according to the leading time of the radiant ray detection pulse discriminated, α, β ray detection pulses can surely be discriminated from the difference of leading time.