Large scale modeling of ultrathin SiO2 films on Si(100) surfaces has been performed using our original potential, which was developed to simulate both Si and SiO2 crystal systems. A SiO2 film was formed by layer-by-layer insertion of oxygen atoms into Si-Si bonds in a Si wafer from one of the surfaces. The thickness of the obtained SiO2 layer was about 17.2 Angstrom, and it showed the presence of the structural transition layer; the average Si-O-Si bond angle becomes smaller in the region closer to the SiO2/Si interface. The peak of Si-O-Si bond angle distribution is shifted toward a narrower angle from the equilibrium angle of 144 degrees, in agreement with experimental results reported so far.