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 provided on the pipe , the first and third transducers converting electrical signals into acoustic signals, the second and fourth transducers 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 provided on the pipe , the first and third transducers converting electrical signals into acoustic signals, the second and fourth transducers 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. <EOS>