The interface state densities near the midgap were measured with the progress of oxidation of atomically flat Si(100) surface. It was found that the interface state distribution in Si bandgap changes periodically with the progress of oxidation. Namely, the interface state density near the midgap of Si decreases drastically at oxide film thickness where the surface roughness of oxide film takes its minimum value, while it does not decrease at oxide film thickness where the surface roughness takes its maximum value. In order to minimize interface state densities, the oxide film thickness should be precisely controlled to within an accuracy of 0.02 nm.