To provide a safe and accurate projection aligner apparatus, which can realize high resolution by preventing an illuminating efficiency and an alignment accuracy from being reduced due to fogging of an essential optical element in the apparatus, immediately discover a cause in the case of a gas state fault, and shorten a down time of the apparatus, and to provide a method for monitoring gaseous state.

The method for monitoring the gas state comprises the steps of fully filling inert gas in a beam shaping optical system 2 and a projection lens 10 via a supply line, providing the optical system 2 and the lens 10 as well as a concentration detectors 13 to 15 for detecting the physical amounts of the gas existing in the peripheral space of the wafer 11, temperature detectors 24 to 26, pressure detectors 27 to 29 and a sensor controller 102, and processing the apparatus, when the detector detects the fault by a main control system 104, a stage drive control system 101, a laser control system 103 and a valve/ flow rate control system 107.