A flow rate measurement device comprising: a pair of ultrasonic wave oscillators (2, 3) provided in a flow path through which a fluid to be measured flows; a transmitting unit configured to drive the ultrasonic wave oscillators (2, 3); a switching unit configured to switche transmission and reception of the ultrasonic wave oscillators (2, 3); an amplifier configured to amplify the ultrasonic wave signal received by each of the ultrasonic wave oscillators (2, 3) to a predetermined amplitude; a reference comparator configured to compare an output of the amplifier with a reference voltage; a reference voltage setting unit configured to adjust and set the reference voltage; a determination unit configured to determine an arrival time of the ultrasonic wave signal from outputs of the reference comparator and the amplifier; a time measuring unit configured to measure a propagation time of transmission and reception of the ultrasonic wave signal from the arrival time of the ultrasonic wave signal determined by the determination unit; and a flow rate calculation unit configured to calculate an instantaneous flow rate of the fluid to be measured based on the propagation time measured by the time measuring unit; characterized in that: the pair of ultrasonic wave oscillators (2, 3) is disposed so as to transmit and receive an ultrasonic wave signal by causing the ultrasonic wave signal to be reflected on an inner wall of the flow path at least once; the flow rate measurement device further comprises: a controller configured to periodically perform adjustment of an amplification rate; and the reference voltage setting unit is configured to adjust the reference voltage if a difference between a previous amplification rate and a current amplification rate is a predetermined value or more, and the instantaneous flow rate calculated by the flow rate calculation unit is a predetermined flow rate or less. a pair of ultrasonic wave oscillators (2, 3) provided in a flow path through which a fluid to be measured flows; a transmitting unit configured to drive the ultrasonic wave oscillators (2, 3); a switching unit configured to switche transmission and reception of the ultrasonic wave oscillators (2, 3); an amplifier configured to amplify the ultrasonic wave signal received by each of the ultrasonic wave oscillators (2, 3) to a predetermined amplitude; a reference comparator configured to compare an output of the amplifier with a reference voltage; a reference voltage setting unit configured to adjust and set the reference voltage; a determination unit configured to determine an arrival time of the ultrasonic wave signal from outputs of the reference comparator and the amplifier; a time measuring unit configured to measure a propagation time of transmission and reception of the ultrasonic wave signal from the arrival time of the ultrasonic wave signal determined by the determination unit; and a flow rate calculation unit configured to calculate an instantaneous flow rate of the fluid to be measured based on the propagation time measured by the time measuring unit; the pair of ultrasonic wave oscillators (2, 3) is disposed so as to transmit and receive an ultrasonic wave signal by causing the ultrasonic wave signal to be reflected on an inner wall of the flow path at least once; the flow rate measurement device further comprises: a controller configured to periodically perform adjustment of an amplification rate; and the reference voltage setting unit is configured to adjust the reference voltage if a difference between a previous amplification rate and a current amplification rate is a predetermined value or more, and the instantaneous flow rate calculated by the flow rate calculation unit is a predetermined flow rate or less. a controller configured to periodically perform adjustment of an amplification rate; and the reference voltage setting unit is configured to adjust the reference voltage if a difference between a previous amplification rate and a current amplification rate is a predetermined value or more, and the instantaneous flow rate calculated by the flow rate calculation unit is a predetermined flow rate or less.