A method for forming an organic mask, includes: permeating an organic solvent into an organic pattern formed on a base film and containing at least one kind of organic material, by contacting the organic pattern with the organic solvent; and thereby, partially or entirely decreasing original adhesion strength between the base film and the organic pattern. A heat treatment may be conducted after contacting to adjust the adhesion strength. Using the organic pattern as a mask, isotropic etching is conducted. As a result, a desired taper angle of the etched base film can be achieved with high accuracy. The taper angle of the etched base film is adjustable by controlling the adhesion strength through the heat treatment.