The physically unclonable functions (PUFs), which ensure the integrity of devices, have been attracted attention as security techniques for internet of things. The ring oscillator (RO) PUF is one of the most popular PUFs. For security issues of PUFs, the risk of modeling analysis is pointed out. To design the secure authentication system using PUFs, it is important to evaluate the resistance against modeling analysis. Therefore, this study proposes a new modeling analysis using genetic algorithm for RO PUF. Moreover, the proposed method introduces a hybrid modeling method combined a hierarchical method and greedy algorithm in order to improve the modeling accuracy. Experiments showed that the proposed method could realize modeling accuracy of 99% with 50,000 training data sets which was 60% of the conventional method.