PURPOSE: To positively detect the peaks of succeeding pulses even if a plurality of pulses continue and converts them from analog to digital data by controlling an A/D converter and a linear gate by detecting the peaks with a peak stretcher.

CONSTITUTION: A control circuit 5 controls an entire timing, controls a peak stretcher 3 according to the discrimination signal of a discrimination circuit 4, and controls an A/D conversion part 6 and a linear gate 2 by detecting the peaks with the peak stretcher 3. Then, the control circuit 5 short-circuits the input line of the discrimination circuit 4 by detecting the peaks of the peak stretcher 3 and at the same time prohibits the input to the peak stretcher 3 and then cancels the prohibition of input to the peak stretcher 3 after going through the A/D conversion part 6, thus positively detecting the peaks of succeeding pulses even if a plurality of pulses continue and then converting the peaks from analog to digital data by the A/D conversion part 6.