An exposure apparatus includes a reticle stage for supporting a reticle, an illumination optical system for illuminating the reticle, a projection optical system for projecting an image of a pattern of the reticle onto a substrate, a polarization state measuring part for measuring a polarization state, and an optical unit, supported by the reticle stage exchangeable for the reticle, for introducing a light from the illumination optical system to the polarization state measuring part, wherein said polarization state measuring part includes a phase shifter, a polarization element, and a light detector for detecting the light from the illumination optical system via the phase shifter and the polarization element.