There is provided a mask forming method, including: forming a photosensitive organic film on a workpiece; generating a first region and a second region in the photosensitive organic film by performing a selective exposure and a post-exposure baking on the photosensitive organic film, the first region having an acidic functional group in the photosensitive organic film, and the second region having a protective group in which the acidic functional group is protected; forming a salt in the first region by causing a basic substance to permeate into the first region using a substance staying in a gaseous state or a solid state; and removing the first region by dissolving the salt in a developer.