A double patterning technology performed by plasma etching of CF4 gas has been known. The double patterning technology uses a wafer which includes a film to be processed, an organic film formed in a plurality of narrow lines on the film to be processed, and a Si oxide film that covers both the respective lines and the film to be processed which is exposed between the lines. In the double patterning technology, the Si oxide film is etched first to expose the respective lines of the organic film and the film to be processed. Then, the exposed organic film is selectively removed. Thereafter, the film to be processed is etched using the remaining Si oxide film as a mask. See, for example, Japanese Patent Application Laid-Open No. 2010-212415.