Abstract: An aligner applies exposure light from an exposure light source to a reticle through an illuminating optical system to project a pattern image of the reticle onto a photosensitive substrate through a projection optical system, and is equipped with a gas supply device for filling spaces formed between optical elements in a lens barrel housing the projection optical system therein with an inert gas, pressure sensors for sensing the pressures in the spaces, and optical performance adjustment devices for adjusting the optical performance of the projection optical system. The optical performance adjustment device can be, for example, a device for adjusting a distance between a pair of the optical elements or a device for adjusting a pressure in a space between the optical elements.