A beam transforming element for setting a polarization state of light which forms a predetermined light intensity distribution on a predetermined surface in an illumination optical system, for use in an illumination optical apparatus by which a surface to be illuminated is illuminated with light supplied from a light source, the beam transforming element comprising: a first basic element made of an optical material with optical activity, for setting a polarization state of first light which forms a first region distribution in the predetermined light intensity distribution; and a second basic element made of an optical material with optical activity, for setting a polarization state of second light which forms a second region distribution in the predetermined light intensity distribution, characterized in that the first basic element and the second basic element, each of which having a crystallographic axis of the optical material along a direction of light transmission, have respective thicknesses different from each other along the direction of light transmission, and that the thickness of the first basic element and the thickness of the second basic element are so set that, when linearly polarized light is incident to the beam transforming element, the light transmitted through the first and second basic elements is set to be linearly polarized light, a direction of polarization of the light transmitted through the first basic element being different from a direction of polarization of the light transmitted through the second basic element, the beam transforming element being configured to form the predetermined light intensity distribution through a diffracting surface of the beam transforming element provided on one side of the first basic element and the second basic element. a first basic element made of an optical material with optical activity, for setting a polarization state of first light which forms a first region distribution in the predetermined light intensity distribution; and a second basic element made of an optical material with optical activity, for setting a polarization state of second light which forms a second region distribution in the predetermined light intensity distribution, characterized in that the first basic element and the second basic element, each of which having a crystallographic axis of the optical material along a direction of light transmission, have respective thicknesses different from each other along the direction of light transmission, and that the thickness of the first basic element and the thickness of the second basic element are so set that, when linearly polarized light is incident to the beam transforming element, the light transmitted through the first and second basic elements is set to be linearly polarized light, a direction of polarization of the light transmitted through the first basic element being different from a direction of polarization of the light transmitted through the second basic element, the beam transforming element being configured to form the predetermined light intensity distribution through a diffracting surface of the beam transforming element provided on one side of the first basic element and the second basic element.