To provide an aligner which can prevent air from flowing into the inside of an airtight space without lowering airtightness, even if elasticity lowers due to deterioration of a sealing member in an airtight space, wherein an illuminating optical member is stored.

An aligner provided with an illuminating optical system and a projection optical system has at least one or more airtight structures, which shut off an optical path from the light source of an illuminating optical system to the mask side end part of a projection optical system from the air. Each airtight structure is constituted of a housing 11, a cover 12 and a sealing member 13 interposed therebetween. A coil spring 16 is inserted between a fastening bolt 14 for fixing the cover 12 to the housing 11 and the cover 12. Thereby, even if elasticity is lowered due to deterioration of the sealing member 13, airtightness of an airtight structure can be maintained by the coil spring 16, thus preventing flow-in of the air, preventing accidents due to suffocation and light amount losses due to the generation of ozone and preventing lowering of illuminating efficiency.