PURPOSE: To measure and detect a precise pulse repetition (PRI) value by updating the PRI value at specific intervals.

CONSTITUTION: The PRI value varies momentarily, so there is some difference between a real PRI value and the PRI output signal 14 of a self-running counter 8. Therefore, a PRI output confirmation signal 10 generated by the signal 14 is inputted to the enable terminal of a PRIn frequency detecting counter 15, which when activated, counts the PRI value. When the counted value coincides with an external specified value 16 of the PRIn frequency, a PRIn frequency detection signal 17 is generated and passed through a waveform shaping circuit 18 to generate an (n)-time PRI data sampling signal 19. Consequently, the PRI value at the time of n×PRI is outputted from a pulse interval measuring device 1 and stored as a PRI set value 5 in the counter 8, which performs counting operation by a clock 7. The signal 19 is generated at specific intervals to allow the real PRI value and signal 14 to enter into follow-up relation, obtaining the precise PRI output signal.