PURPOSE: To determine true counting rate by keeping the dead time constant until the subsequent pulse is detected after the arrival of the first pulse to facilitate the calculation of the probability of missed countings.

CONSTITUTION: A pulse as shown by (m) is applied to a signal (i) and the time range for the portion of the lower limit level Vl thereof is set at τ0. When the output (g) of an upper limit comparator CM1 is not sent within the time length τ1 half the time range τ0 after an FF circuit DF1 operates by the output (a) of a lower limit comparator CM2, the height value of any pulse exists between the upper and lower limit levels Vh and Vl. Therefore, a resistance R1 and the capacitance C1 are so set that the time lag from the operation of the circuit DF1 to the operation of a multivibrator MM1 corresponds to the time τ1 and so the component pulse n1 even in a pulse as shown by n1 and n2 can be detected. If the time range τ3 as the sum of the time range τ2 and the time range τ1 is set larger than the time range τ0 by the resistance R1 and the capacitance C1, the signal of the component pulse p2 will not be sent even if a pulse as shown by p1 and p2 is inputted.