A flow rate measuring device, comprising: a pair of transducers 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 ; 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 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 ; 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. <EOS>