An exposure apparatus for printing, by exposure, a pattern of an original onto a substrate includes a housing tightly filled with a predetermined ambience and for accommodating therein at least a portion of an exposure light optical axis, and a detection system having an optical system, wherein a portion of a light path of the detection system is disposed in a first space enclosed by the housing, and wherein at least another portion of the detection system including an electrical element thereof is disposed in a second space outside the housing.