A flow rate measurement apparatus comprising: at least two transducers (1a, 1b) provided at different positions in a longitudinal direction of a pipe, the at least two transducers (1a, 1b) including first and second transducers (1a, 1b), wherein each of the first and second transducers (1a, 1b) that converts an electric signal into an ultrasonic signal or converts an ultrasonic signal into an electric signal, wherein the flow rate measurement apparatus transmits an ultrasonic signal to at least one of the first and second transducers (1a, 1b) to another of the first and second transducers (1a, 1b) as a transmission signal, receives a received signal, separates a predetermined first target signal based on a correlation coefficient between the received signal and a predetermined reference wave signal, and thereafter, measures a flow rate of a fluid inside the pipe based on the first target signal, characterized in that the flow rate measurement apparatus further comprising: a transmitter (2, 20) that transmits a transmission signal, which includes an ultrasonic signal having a band wider than the ultrasonic signal, or a plurality of ultrasonic signals having a plurality of different frequency ranges; and a receiver (3, 4, 5) that separates a second target signal passing through the fluid from the received signal by utilizing that an attenuation rate of the ultrasonic signal in the fluid in a predetermined frequency band is different from an attenuation rate of the ultrasonic signal in the pipe in the frequency band, and measures a flow rate based on a separated second target signal, in the same manner as that of measuring the flow rate of the fluid inside of the pipe based on the first target signal. at least two transducers (1a, 1b) provided at different positions in a longitudinal direction of a pipe, the at least two transducers (1a, 1b) including first and second transducers (1a, 1b), wherein each of the first and second transducers (1a, 1b) that converts an electric signal into an ultrasonic signal or converts an ultrasonic signal into an electric signal, wherein the flow rate measurement apparatus transmits an ultrasonic signal to at least one of the first and second transducers (1a, 1b) to another of the first and second transducers (1a, 1b) as a transmission signal, receives a received signal, separates a predetermined first target signal based on a correlation coefficient between the received signal and a predetermined reference wave signal, and thereafter, measures a flow rate of a fluid inside the pipe based on the first target signal, characterized in that the flow rate measurement apparatus further comprising: a transmitter (2, 20) that transmits a transmission signal, which includes an ultrasonic signal having a band wider than the ultrasonic signal, or a plurality of ultrasonic signals having a plurality of different frequency ranges; and a receiver (3, 4, 5) that separates a second target signal passing through the fluid from the received signal by utilizing that an attenuation rate of the ultrasonic signal in the fluid in a predetermined frequency band is different from an attenuation rate of the ultrasonic signal in the pipe in the frequency band, and measures a flow rate based on a separated second target signal, in the same manner as that of measuring the flow rate of the fluid inside of the pipe based on the first target signal. a transmitter (2, 20) that transmits a transmission signal, which includes an ultrasonic signal having a band wider than the ultrasonic signal, or a plurality of ultrasonic signals having a plurality of different frequency ranges; and a receiver (3, 4, 5) that separates a second target signal passing through the fluid from the received signal by utilizing that an attenuation rate of the ultrasonic signal in the fluid in a predetermined frequency band is different from an attenuation rate of the ultrasonic signal in the pipe in the frequency band, and measures a flow rate based on a separated second target signal, in the same manner as that of measuring the flow rate of the fluid inside of the pipe based on the first target signal.