To quantize the position and size of bubbles in an aligner using an immersion method.

The aligner includes an illumination optical system for illuminating a reticle with a light flux from a light source, and a projection optical system for projecting the pattern of the reticle on a substrate. The aligner projects the pattern on the substrate via an immersion liquid filled in a gap between an optical element arranged closest to the substrate concerning the projection optical system and the substrate. The aligner includes a bubble observation system for detecting the bubbles existing in the immersion liquid filled in the gap via the projection optical system.