A flow rate measuring device comprising: a first transducer (2, 22) and a second transducer (3, 23) disposed in a flow path (1, 21) through which fluid to be measured flows, and for transmitting/receiving an ultrasonic signal; a transmitter (5, 25) configured to drive the first transducer (2, 22) and the second transducer (3, 23); a switching part (4, 24) configured to switch transmission/reception between the first transducer (2, 22) and the second transducer (3, 23); an amplifier (6, 26) configured to amplify a signal received by the first transducer (2, 22) and the second transducer (3, 23); a reference comparator (7, 27) configured to compare between an output from the amplifier (6, 26) and a reference voltage; a reference-voltage setting part configured to set the reference voltage to be used for comparison in the reference comparator (7, 27); a determining part (8, 28) configured to determine an arrival time of the ultrasonic signal based on an output of the reference comparator (7, 27) and the output of the amplifier (6, 26); a timer (9, 29) configured to calculate a propagation time from the transmission to the reception of the ultrasonic signal based on the arrival time determined by the determining part (8, 28); a controller (10, 30) configured to calculate at least one of a flow velocity and a flow rate of the fluid to be measured according to the propagation time calculated by the timer (9, 29); and a reference-voltage changing part configured to change the reference voltage to be set in the reference-voltage setting part, wherein the reference-voltage changing part is configured to change the reference voltage according to an amplification degree employed in the amplifier (6, 26); characterized in that: the reference-voltage changing part is configured to change the reference voltage as a linear function of the amplification degree. a first transducer (2, 22) and a second transducer (3, 23) disposed in a flow path (1, 21) through which fluid to be measured flows, and for transmitting/receiving an ultrasonic signal; a transmitter (5, 25) configured to drive the first transducer (2, 22) and the second transducer (3, 23); a switching part (4, 24) configured to switch transmission/reception between the first transducer (2, 22) and the second transducer (3, 23); an amplifier (6, 26) configured to amplify a signal received by the first transducer (2, 22) and the second transducer (3, 23); a reference comparator (7, 27) configured to compare between an output from the amplifier (6, 26) and a reference voltage; a reference-voltage setting part configured to set the reference voltage to be used for comparison in the reference comparator (7, 27); a determining part (8, 28) configured to determine an arrival time of the ultrasonic signal based on an output of the reference comparator (7, 27) and the output of the amplifier (6, 26); a timer (9, 29) configured to calculate a propagation time from the transmission to the reception of the ultrasonic signal based on the arrival time determined by the determining part (8, 28); a controller (10, 30) configured to calculate at least one of a flow velocity and a flow rate of the fluid to be measured according to the propagation time calculated by the timer (9, 29); and a reference-voltage changing part configured to change the reference voltage to be set in the reference-voltage setting part, wherein the reference-voltage changing part is configured to change the reference voltage according to an amplification degree employed in the amplifier (6, 26);