When a flow rate is equal to or more than a reference flow rate, a reference voltage is changed to a level capable of measuring stably to thereby realize the improvement of measurement accuracy at the time of a large amount of flow rate. A flow rate determination means compares a flow rate calculated by a flow rate calculation means with the reference flow rate. When the calculated flow rate is larger than the reference flow rate, the reference voltage set by a reference voltage setting means is changed, whereby the zero cross point can be detected stably at the portion where the degree of the influence of the change in the amplitude of the received signal is small. Thus, it is possible to provide a flow rate measuring device capable of measuring a flow rate stably with high accuracy even when a fluid flow is disturbed due to a large amount of flow rate.