To obtain a circuit for measuring the width of pulseless than 100nsec automatically and accurately at high speed without requiring any resetting for the pulse having different width.

The pulse width measuring circuit comprises a comparison circuit 12 receiving pulses 20, 30 for which the duration is measured and activating a pulse existence signal 76 in response thereto, a counter counting the pulses in response to the pulse existence signal and outputting a pulse count indicative of the duration of received pulse, and reset circuits 88, 90, 92 for generating a reset signal in response to the front edge of pulse existence signal and resetting the counter automatically to a reference count in response to the front edge of pulse existence signal.