An oxidizing method and oxidizing apparatus in which a plasma generating chamber having an oxidizing gas supply port and a substrate processing chamber having an exhaust port and internally having a substrate susceptor are connected via a partition having a number of through holes, a plasma of an oxidizing gas supplied into the plasma generating chamber is generated, and an oxide layer is formed on a substrate surface by supplying the generated active species onto a substrate are characterized in that the partition is connected to a power supply via a switching mechanism such that a positive, negative, or zero voltage is applied to the partition, and an oxidation process is performed by changing the ratio of radicals, positive ions, and negative ions in the active species supplied onto the substrate by switching the voltages at least once during the oxidation process.