PURPOSE: To remove an inessential scum other than a mask before an etching operation, to enhance a working accuracy and to prevent the damage by a plasma and the contamination with impurities both of which are caused when the scum is removed.

CONSTITUTION: A high-output and low-pressure mercury lamp 5 which emits utraviolet rays at wavelengths of 185nm and 254nm is provided; the ultraviolet rays are reacted with O2 in the air; O2 (at 185nm)→O3 and O3 (at 245nm)→O*+O2 (where 0* is active oxygen). In addition, the bond of an organic materialbased scum 4 is cut by the ultraviolet rays; C-H→C,H. Then, the scum 4 is reacted with the active oxygen and changed to volatile molecules. C,H+O*→CO2↑, H2O↑. The scum 4 is removed by a mechanism explained above.