According to one embodiment, an opening pattern is formed in the core film above a processing target, and a mask film is conformably formed above the processing target. Next, etch-back of the mask film is performed so that the mask film remains on a side surface of the core film. After that, line-and-space shaped core patterns, made of the core film, is formed in an area other than an area forming the opening pattern. Next, sidewall patterns are formed around the core patterns, and the core patterns are removed. Next, the processing target is patterned by using the mask film and the sidewall patterns.