A flow-rate measuring apparatus for measuring a flow rate of a fluid inside a pipe having a predetermined cross-sectional area, wherein the flow-rate measuring apparatus is connected to a first and second transducers (2, 3) provided at different positions in the pipe, respectively, the first transducer (2, 3) converting an electrical signal into an acoustic signal, the second transducer (2, 3) converting an acoustic signal into an electrical signal, wherein the flow-rate measuring apparatus is configured to: transmit a measurement signal having a plurality of frequencies and a time length by the first transducer (2, 3), and receive the measurement signal by the second transducer (2, 3) through the fluid inside the pipe, calculate a first correlation coefficient between a reference signal corresponding to the transmitted measurement signal, and the received measurement signal, and calculate the flow rate of the fluid inside the pipe based on the measurement signal, when a peak value of the first correlation coefficient is higher than a first threshold, and characterized in that the flow-rate measuring apparatus is further configured to retransmit the measurement signal with changing the time length of the measurement signal, when the peak value of the first correlation coefficient is equal to or lower than the first threshold. wherein the flow-rate measuring apparatus is connected to a first and second transducers (2, 3) provided at different positions in the pipe, respectively, the first transducer (2, 3) converting an electrical signal into an acoustic signal, the second transducer (2, 3) converting an acoustic signal into an electrical signal, wherein the flow-rate measuring apparatus is configured to: transmit a measurement signal having a plurality of frequencies and a time length by the first transducer (2, 3), and receive the measurement signal by the second transducer (2, 3) through the fluid inside the pipe, calculate a first correlation coefficient between a reference signal corresponding to the transmitted measurement signal, and the received measurement signal, and calculate the flow rate of the fluid inside the pipe based on the measurement signal, when a peak value of the first correlation coefficient is higher than a first threshold, and characterized in that the flow-rate measuring apparatus is further configured to retransmit the measurement signal with changing the time length of the measurement signal, when the peak value of the first correlation coefficient is equal to or lower than the first threshold. transmit a measurement signal having a plurality of frequencies and a time length by the first transducer (2, 3), and receive the measurement signal by the second transducer (2, 3) through the fluid inside the pipe, calculate a first correlation coefficient between a reference signal corresponding to the transmitted measurement signal, and the received measurement signal, and calculate the flow rate of the fluid inside the pipe based on the measurement signal, when a peak value of the first correlation coefficient is higher than a first threshold, and characterized in that the flow-rate measuring apparatus is further configured to retransmit the measurement signal with changing the time length of the measurement signal, when the peak value of the first correlation coefficient is equal to or lower than the first threshold.