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 first, second, third, and fourth transducer (2,3,4,5) provided on the pipe, the first and third transducers (2, 4) converting electrical signals into acoustic signals, the second and fourth transducers (3, 5) converting acoustic signals into electrical signals; wherein the flow-rate measuring apparatus is configured to: transmit a first measurement signal having at least one first frequency by the first transducer, and receive the first measurement signal by the second transducer through the fluid inside the pipe; determine a second frequency based on the first measurement signal; transmit a second measurement signal having the second frequency by the third transducer toward an interface between the pipe and the fluid, and receive the second measurement signal reflected at the interface by the fourth transducer; calculate the flow rate of the fluid inside the pipe so as to reflect a viscosity of the fluid based on the first and second measurement signals; and the flow-rate measuring apparatus is further configured to: calculate a first velocity of the fluid based on the first measurement signal; calculate a shear rate of the fluid based on the first velocity of the fluid; and determine the second frequency so as to be proportional to the shear rate of the fluid. wherein the flow-rate measuring apparatus is connected to first, second, third, and fourth transducer (2,3,4,5) provided on the pipe, the first and third transducers (2, 4) converting electrical signals into acoustic signals, the second and fourth transducers (3, 5) converting acoustic signals into electrical signals; wherein the flow-rate measuring apparatus is configured to: transmit a first measurement signal having at least one first frequency by the first transducer, and receive the first measurement signal by the second transducer through the fluid inside the pipe; determine a second frequency based on the first measurement signal; transmit a second measurement signal having the second frequency by the third transducer toward an interface between the pipe and the fluid, and receive the second measurement signal reflected at the interface by the fourth transducer; calculate the flow rate of the fluid inside the pipe so as to reflect a viscosity of the fluid based on the first and second measurement signals; and the flow-rate measuring apparatus is further configured to: calculate a first velocity of the fluid based on the first measurement signal; calculate a shear rate of the fluid based on the first velocity of the fluid; and determine the second frequency so as to be proportional to the shear rate of the fluid. transmit a first measurement signal having at least one first frequency by the first transducer, and receive the first measurement signal by the second transducer through the fluid inside the pipe; determine a second frequency based on the first measurement signal; transmit a second measurement signal having the second frequency by the third transducer toward an interface between the pipe and the fluid, and receive the second measurement signal reflected at the interface by the fourth transducer; calculate the flow rate of the fluid inside the pipe so as to reflect a viscosity of the fluid based on the first and second measurement signals; and the flow-rate measuring apparatus is further configured to: calculate a first velocity of the fluid based on the first measurement signal; calculate a shear rate of the fluid based on the first velocity of the fluid; and determine the second frequency so as to be proportional to the shear rate of the fluid. calculate a first velocity of the fluid based on the first measurement signal; calculate a shear rate of the fluid based on the first velocity of the fluid; and determine the second frequency so as to be proportional to the shear rate of the fluid.