Among the numerous gas concentration measurement methods, ultrasonic methods are desirable because it can measure a greenhouse or ozone layer-depleting gases that are used as refrigerant gases without extracting the gases and facilitate the measurement of successive gas concentration changes. An ultrasonic gas sensor was successfully fabricated using the difference in sound velocity between polyatomic gases (such as hydrofluorocarbons as refrigerant gases) and base gases. Each gas has its own specific sound velocity; therefore, the traveling time between a transmitter and receiver also has a unique value depending on the type of gas. The gas concentration can be calculated from the difference in the time of flight. The sound velocity in gas is theoretically defined; thus, the gas concentration can be accurately and quickly (in microseconds) calculated from precise sound-traveling-time measurements. Polyatomic gases have specific heat ratios different from those of diatomic gases, such as hydrogen, oxygen, and nitrogen. This paper presents a new algorithm capable of calculating the concentration of polyatomic gases, which have molecular weights and specific heat ratios that differ from those of the base gas. A model-based design was used for this complex gas concentration calculation based on the numerical analysis of nonlinear equations in mixtures of polyatomic molecule gases and diatomic molecule base gases.