A light shielding unit, which shields a part of an exposure light with a light shielding member, includes a sensor which detects displacement of the light shielding member, a driving device which drives the light shielding member based on a detection result obtained by the sensor, a case which accommodates the sensor and the driving device, and a bellows mechanism which gas-seals the case with respect to the light shielding member. When the illumination area of the illumination light is defined, it is possible to mitigate the influence of the foreign matters and/or the heat generated from the driving mechanism.