To provide a method for preparing an oxide film capable of treating base materials each having an electrically isolated pattern in a bulk without using an advanced device.

Regarding the method where, through energizing between two electrodes 2, 3 dipped into an electrolytic solution, an oxide film is prepared on the body 4 to be treated, the surface of the body 4 to be treated comprises a circuit pattern composed of an insulation part and an electrically conductive part, and, a stage where the body 4 to be treated is dipped into an electrolytic solution in such a manner that it is not contacted with the electrodes, and a stage where the electrically conductive part is oxidized are comprised.