Indium tin oxide (ITO) films were prepared by a planar magnetron sputtering system with facing ITO ceramic targets. In this case, the substrate is placed between the targets, so that film bombardment by energetic particles such as O- ions and O atoms can be avoided. With the addition of small partial pressure of O-2 in the atmosphere, film resistivity shows a minimum. A resistivity of 1.6 X 10(-4) Ohm cm is attained. This value is nearly the same as the lowest value reported until now, which confirms that film preparation by the facing target system is effective in decreasing the film resistivity. Adding a greater proportion of O-2 gas in the Ar induces a decreased carrier concentration, which is due to the oxidation of Sn donor atoms and oxygen vacancies. The oxidation mechanism is discussed.