In order to measure a propagation time of ultrasonic signals, this flow quantity measuring apparatus has an amplifying unit (6) for amplifying received signals, a reference voltage changing unit (11), a reference voltage setting unit (12), and a reference comparing unit (7) for comparing the received waveform and a reference voltage with each other. The reference voltage changing unit (11) changes the reference voltage corresponding to the amplification degree of the received waveform amplified by means of the amplifying unit (6), and stably measures the propagation time by comparing the changed reference voltage and the received waveform with each other. Consequently, measurement accuracy of the flow rate, flow quantity, and the like of a fluid to be measured can be prevented from deteriorating.