PURPOSE: To enhance analytical accuracy by mounting an upper limit counter and a lower limit counter respectively counting the selection outputs of upper and lower limit selecting circuits and further mounting an operator for calculating the difference between the outputs of both counters.

CONSTITUTION: In a peak selector 1, the detection pulse amplified by a proportional amplifier 2 is respectively inputted to an upper limit comparator 4a and a lower limit comparator 4b. The upper comparator 4a selects only the detection pulse whose peak value exceeds an upper limit value to output the same. The lower limit comparator 4b selects only the detection pulse wherein the peak value exceeds a lower limit value to output the same. The output pulse of the upper limit comparator 4a and that of the lower limit comparator 4b are respectively counted for a definite period by upper and lower limit counters 6a, 6b. The outputs of both counters 6a, 6b counted for the definite period are inputted to the operator 8 of the next stage to calculate the difference between the count values. By this method, only the detection pulse having the peak value between the upper and lower limit values is selected and the enhancement of analytical accuracy can be achieved.