The growth mechanism of SiO2 thin film on Si(100) and Si(111) by ozone was investigated using various surface/interface analytical techniques such as X-ray photoelectron spectroscopy (XPS), second harmonic generation (SHG) and medium energy ion scattering spectroscopy (MEIS). Two different ozone generators were fabricated and used for the investigation. The first ozone generator, which was used for the study of initial oxidation, supplies low pressure (< 10(-2) Pa) and high purity (> 80%) ozone gas by vaporization of pure liquid ozone at low temperature (< 100 K). The second ozone generator, used mainly for ultrathin SiO2 film growth, supplies high pressure (1 atm) ozone gas with concentration < 30% by desorbing ozone adsorbed on silica-gel. Through the comparison of ozone oxidation to the oxidation with molecular oxygen, followings features of the ozone oxidation were made clear. (i) Atomic oxygen dissociated from ozone molecules at Si surface directly attacks the back bond of Si, hence it can oxidize hydrogen-terminated Si which oxygen molecules cannot. (ii) The oxide thin film growth proceeds in layer-by-layer manner, especially at the initial stage of oxidation. (iii) Formation of suboxids at and/or near the SiO2/Si interface was suppressed, leading to a stable Si-O-Si network formation even at low pressure and low temperature condition. In addition to these features, the existence of no (or very thin) structural transition layer was suggested for ozone oxide film from MEIS experiments and etching experiment with dilute HF solution, while those experiments for thermally grown oxide showed the existence of the transition layers with thickness of approximately 1 nm.