PURPOSE: To permit correction of fluctuation in the width of resist patterns by subjecting the residue of the resist on a mask glass substrate to a descum treatment by a plasma beam which varies the intensity according to the distribution condition of the residue amt.

CONSTITUTION: The surface of a resist film 3 subjected to exposing and developing stages is scanned over the entire part by the plasma beam 11 sufficiently controlled in the amt. of gas and speed at every place in accordance with the distribution data on the prediction for the fluctuation in the width of the mask patterns, i.e., resist patterns 3a, 3b on the resist film 3 previously formed by actual measurements. The distribution data on the prediction for the fluctuation in the width of the mask patterns may be statistically determined estimation values or individually actually measured values. Namely, the surface of the resist film is kept scanned successively while the intensity of the plasma beam 1 is changed according to the fluctuation rates of the resist patterns 3a, 3b. The fluctuation in the width of the resist patterns is thereby suppressed down to the naturally generated fluctuation.