PURPOSE: To contrive improvement of liquid scintillation of a low level by simultaneous detection of an optical event and comparison and processing of counted pulse numbers.

CONSTITUTION: A liquid protection scintillation unit 10 containing is set the shielding area of photomultipliers 12, 14 and when the radioactive nuclides in the unit 10 decays, beta particles are emitted to excite fluorescent material. The fluorescent material converts energy from the beta particles into low energy photons, the photomultipliers 12, 14 detect them to generate an electric pulse. Photoelectric detectors 16, 18 convert it into a digital pulse to be supplied to coincidence detector 20 and an OR gate 22. When the detector 20 detects that the pulses have been simultaneously generated in both the photomultipliers, coincidence signal is transferred to a coincidence latch 24. If the range of the background event is determined by the simultaneous detection of an optical event and comparison and processing of pulse numbers, scintillation can be discriminated between an event caused by a specimen radiation and another event caused by a background radiation.