The SGP4 model is one of the analytical orbit models applied for the orbit prediction using TLE orbit information. For the orbit accuracy improvement, a model conversion was performed to apply a more precise model. Assuming that a TLE data set is the best possible fit to the observation data, the orbital arc in a specified time span was reconstructed from several TLE sets. Using osculating elements of the SGP4 output as measurements data, the orbit determination as well as the orbit propagation were performed with the numerical orbit model. The performance of this process was assessed by a comparison with precise orbit information. After the inherent model accuracy analysis using the best-fitted TLE based on GPS data, the process was applied to the publically available TLE, where an accuracy improvement was achieved for some objects. The same process was also successfully applied to re-entry prediction calculations. The more advanced process needs to be studied for a general application.