A method MT according to an embodiment provides a technique capable of controlling a pattern shape during processing of an organic film and the like. A wafer W as an object to which the method MT in the embodiment is applied includes an etching target layer EL, an organic film OL, and a mask ALM, the organic film OL is constituted by a first region VL1 and a second region VL2, the mask ALM is provided on the first region VL1, the first region VL1 is provided on the second region VL2, and the second region VL2 is provided on the etching target layer EL. In the method MT, the first region VL1 is etched to reach the second region VL2 by generating a plasma of a gas containing nitrogen gas in the processing container 12 in which the wafer W is accommodated, a mask OLM1 is formed from the first region VL1, a protective film SX is conformally formed on a side surface SF of the mask OLM1, the second region VL2 is etched to reach the etching target layer EL to form a mask OLM2 from the second region VL2.