To provide an illumination optical system capable of stably keeping an optical characteristic of an optical member like, such as a diffractive optical element.

The illumination optical system includes a light-transmitting optical system 1, 2 transmitting luminous flux from a light source LS to an effective diffractive optical plane of a diffractive optical element 3a. The light-transmitting optical system includes a distribution formation member 2 forming, in a specific region, a second light intensity distribution where the light intensity at the center is lower than that of a first light intensity distribution and peripheral light intensity is higher than that of the first light intensity distribution, based on non-parallel luminous flux having entered in a state of the first light intensity distribution where the light intensity at the center is higher than that in the periphery. The distribution formation member is structured such that a light ray group entering by forming a specific angle with an optical axis AX forms a light intensity distribution having a central region low in light intensity relative to that of the circumference at a position in accordance with the specific angle in the effective diffractive optical plane.