ALIGNER

PROBLEM TO BE SOLVED: To guide exposure light to an object with high efficiency when supplying a gas having a high transmittivity against the exposure light to at least a part of an optical path of the exposure light. SOLUTION: A main body 26 of an aligner is installed inside an environment chamber 7 on a floor F1 and most parts of an illumination optical system from a cover 1 housing an outside F2 laser light source 3 to the inside of the environment chamber 7 are covered with a sub-chamber 6. A high purity helium gas inside a bomb 32 on a downstairs floor F2 is supplied to the sub-chamber 6 and a projection optical sytem PL which is the main body 26 of the aligner through a pipe 31 to increase a transmittivity against the exposure light. Helium gas is also used for a gas for an XY stage 23 placed on a surface plate 24 by a gas bearing method to position a wafer W which is an object to be exposed and a gas for damping stands 25A, 25B for supporting the surface plate 24.