A scanning projection exposure apparatus transfers a pattern of a mask onto a photosensitive substrate while moving the photosensitive substrate along a scanning direction. The scanning projection exposure apparatus can include an illumination optical system optically connected to a light source, and a projection optical system arranged in an optical path between the mask and the substrate. The illumination optical system illuminates the mask with linearly polarized light whose polarization direction extends along one of the scanning direction and a direction perpendicular to the scanning direction, without losses in quantity of light. The illumination optical system of the scanning projection exposure apparatus also can include a wave plate which is rotatable about a traveling direction of the light.