PURPOSE: To provide a trigger device having the capability of completely preventing the occurrence of malfunction while setting a short measurement time for following up a natural phenomenon, and ensuring a burst to be properly detected, regarding a trigger device for measuring a random phenomenon in the natural world and detecting bursted state.

CONSTITUTION: The occurrence of radiation or the like is measured repeatedly over the prescribed time in a counting circuit 2, and the results of the measurement are compared in a comparator 4 to process the results on the basis of the Poisson's distribution. The comparison result is flagged in a shift register 5 and saved. Then, the number of flags over a plurality of consecutive times is measured with a flag check circuit 6, and processed on the basis of the binominal distribution for comparison with a reference flag number established with a flag comparator 7. As a result, the probability where the flag number in the shift register 5 is equal to or above the prescribed value, can be made zero and, therefore, the occurrence of malfunction can be completely prevented.