A flow rate measuring device, comprising: a pair of transducers (2, 3) each of which is provided at a fluid path and transmits and receives an ultrasonic signal; a transmission means which drives the transducers; a switching means which switches transmission/reception between the transducers (2, 3); an amplifying means which amplifies a received signal from the transducer; a reference comparison means which compares an output from the amplifying means with a reference voltage; a reference voltage setting means which sets the reference voltage to be compared by the reference comparison means; a determination means which determines an arrival timing of the ultrasonic signal based on an output from the reference comparison means and the output from the amplifying means; a clock means which measures a propagation time of the ultrasonic signal from the transducer on a transmission side to the transducer on a reception side; and a calculation means which calculates at least one of a flow speed and a flow rate based on the time calculated by the clock means, wherein the reference voltage setting means changes the reference voltage when the flow speed or the flow rate calculated by the calculation means is larger than a preset reference flow speed or a preset reference flow rate or when a degree of a variance of the flow speed or the flow rate calculated by the calculation means is larger than a reference variance set in advance, characterized in that an initial value of the reference voltage is set to a voltage capable of detecting an n-th waveform of the received waveform amplified by the amplifying means, and the reference setting means changes the reference voltage to a voltage capable of detecting an n-1-th waveform just before the n-th waveform, n being larger than 1. a pair of transducers (2, 3) each of which is provided at a fluid path and transmits and receives an ultrasonic signal; a transmission means which drives the transducers; a switching means which switches transmission/reception between the transducers (2, 3); an amplifying means which amplifies a received signal from the transducer; a reference comparison means which compares an output from the amplifying means with a reference voltage; a reference voltage setting means which sets the reference voltage to be compared by the reference comparison means; a determination means which determines an arrival timing of the ultrasonic signal based on an output from the reference comparison means and the output from the amplifying means; a clock means which measures a propagation time of the ultrasonic signal from the transducer on a transmission side to the transducer on a reception side; and a calculation means which calculates at least one of a flow speed and a flow rate based on the time calculated by the clock means, wherein the reference voltage setting means changes the reference voltage when the flow speed or the flow rate calculated by the calculation means is larger than a preset reference flow speed or a preset reference flow rate or when a degree of a variance of the flow speed or the flow rate calculated by the calculation means is larger than a reference variance set in advance, characterized in that an initial value of the reference voltage is set to a voltage capable of detecting an n-th waveform of the received waveform amplified by the amplifying means, and the reference setting means changes the reference voltage to a voltage capable of detecting an n-1-th waveform just before the n-th waveform, n being larger than 1.