PURPOSE: To obtain a photomask for forming a minute pattern of high accuracy for a semiconductor device, etc. by forming a light shielding film on a light transmitting substrate, coating the film with a resist film having a predetermined pattern, implanting ions into the shielding film, and carrying out gas plasma etching.

CONSTITUTION: On transparent substrate 1 of glass or the like a metal single- layer film such as a Cr film, a multilayer film or a metal oxide film is formed as light shielding film 2. After forming positive type electron beam resist film 4 of polybutene-1-sulfone (PBS) or the like on film 2, a predetermined pattern is drawn with electron beams and developed to obtain PBS film 4a having transmitting parts 41. Ions of an element such as Sb, W, Ti or Mo which forms a high m.p. compound by oxidation are then implanted, and gas plasma etching is carried out with a gas contg. CCl4 or the like. Film 4a and unimplanted film 2 are removed in succession, and film 2a with plasma etching resistance remains. Thus, a minute pattern of high accuracy is formed.