There is provided a method for processing an inner wall surface of a micro vacancy, capable of reliably etching and cleaning even if the hole provided to the substrate to be processed is narrow and deep. The substrate has a surface on which a processing solution is to be applied and a micro vacancy with an opening on the surface. An aspect ratio (l/r) of the micro vacancy being at least 5, or the aspect ratio being less than 5 and a ratio (V/S) of a micro vacancy volume (V) to a surface area of the opening (S) being at least 3. The substrate is arranged in a processing space. Next, the processing space is depressurized, and subsequently the processing solution is introduced into the processing space so as to process the inner wall surface of the micro vacancy.