A flow-rate measuring apparatus transmits a first measurement signal having at least one first frequency by a first transducer, and receives the first measurement signal by a second transducer through a fluid inside a pipe. The flow-rate measuring apparatus determines a second frequency based on the first measurement signal. The flow-rate measuring apparatus transmits a second measurement signal having the second frequency by a third transducer toward an interface between the pipe and the fluid, and receives the second measurement signal reflected at the interface by a fourth transducer. The flow-rate measuring apparatus calculates a 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.