To measure the propagation time of an ultrasonic signal, the flow rate measuring device of the present invention has amplifier for amplifying a received signal, reference-voltage changing part, reference-voltage setting part, and reference comparator for comparing between the received signal waveform and reference voltage. Reference-voltage changing part changes the reference voltage according to the amplification degree of the amplified waveform of the received signal in amplifier. Comparing the changed reference voltage with the received signal waveform enhances stability in calculation of the propagation time. This prevents measurement accuracy of the flow velocity and the flow rate of fluid to be measured from degradation.