A nozzle throat erosion problem occurs in developing a 15 kN-thrust class motor. To obtain a history of the nozzle throat area in a hybrid rocket static firing test, a new method is developed. Although the specific heat ratio of the combustion gas, which depends on the oxidizer to fuel ratio ξ, is necessary to calculate a nozzle throat area, it is difficult to obtain temporal ξ in hybrid rockets. A reconstruction technique, which estimates temporal ξ, needs chamber pressure, oxidizer flow rate, and nozzle throat area as input data. These two equations are solved simultaneously to acquire two convergence calculations for nozzle throat area and ξ. The new method was applied to a static firing test. The results show a typical erosion history, showing the validity of this method.