PURPOSE: To etch a film to be milled with a high accuracy by reverse tapering the side wall surfaces of a cover at the time of adhering this cover as a mask to the surface of this film to be milled and ion milling the film.

CONSTITUTION: The cover 2 is adhered as the mask to the surface of the film 1 to be milled on a supporting layer 3 on which an insulating film is formed. The outer side regions of the cover exclusive of the regions covered by the cover 2 are ion milled to form the patterns. Both side walls 23, 24 between the base 21 of the cover 2 in contact with the film 1 to be milled and the surface 22 on the opposite side are formed as reverse tapered surfaces. The particles splashed when the film is etched by the ion milling do not, therefore, grow to be continuous on the surface of the film 1 to be milled from the side wall surfaces 23, 24 even if these particles adhere to the side wall surfaces 23, 24 of the cover and form a readhered film. As a result, the readhered film is removed together with the cover when the cover 2 is removed. The projections by the readhesion does not remain and the patterns are formed with the high accuracy.