We have investigated ultrathin silicon oxide film growth by highly concentrated ozone at atmospheric pressure. Oxide film >2 nm was grown on as-received Si(100) even at room temperature. The etching rate by dilute hydrofluoric acid solution of oxide fabricated on Si(100) at 350 degrees C by this method was almost the same as that of thermally grown oxide so that him density is equivalent to that of thermally grown oxide. The etching rate of this film also shows no dependence on the film depth. This is indicating that the transition layer due to the lattice mismatch of substrate and oxide is limited within a thinner region than that of thermally grown oxide. It also indicates that an oxide film with higher film density can be synthesized on the surface with preoxide film already formed to protect bare substrate surfaces.