An exposure apparatus comprises a light source emitting light of the ultraviolet region; an illuminating system for introducing the light from the light source into a mask bearing a predetermined pattern; an exposure unit for forming the image of the mask pattern on a photosensitive substrate; a chamber accommodating the light source, the illuminating system and the exposure unit; and an impurity eliminating device for eliminating at least an organic impurity inducing a photochemical reaction with the light of the ultraviolet region.