A controllability of a size of a mask can be improved in a multi-patterning method. A process of forming a silicon oxide film on a first mask and an antireflection film is performed. In this process, plasma of a first gas including a silicon halide gas and plasma of a second gas including an oxygen gas are alternately generated. Then, a region of the silicon oxide film is removed such that only a region along a side wall of the first mask is left, and then, the first mask is removed and the antireflection film and an organic film is etched.