In an ultrasonic flow meter, it is desired that the operation of the vibrator used for measuring the propagation time of an ultrasonic wave is intermittent for saving an electric power. However, in a conventional ultrasonic flow meter, there is such a problem that since it takes a time for stabilization of the pulses of the vibrator, the intermittent operation of the vibrator cannot be performed.This ultrasonic flow meter comprises a vibrator (60) and a vibration starting means (61) for quickening the vibration of the vibrator. The pulses of the vibrator vibrated by the vibration starting means (61) are used for measuring the propagation time of an ultrasonic wave.