To provide: an illumination optical system accurately forming an effective light source shape; an exposure device; and a device manufacturing method.

The illumination optical system illuminating an illumination object surface by using luminous flux from a light source includes: a first optical element forming a light intensity distribution having a multipolar shape at a position having a relation of Fourier transform with the illumination object surface; a second optical element specifying an angle distribution of luminous flux entering into the first optical element; a capacitor optical system converging luminous flux having passed through the first optical element; and a pair of conical prisms where one-side prism has a conical concave surface and a flat surface, the other-side prism has a conical convex surface and a flat surface, half apex angles of the respective prisms are equal to each other, and the distance between the respective prisms is variable. When it is assumed that the half apex angle, an angle , and an opening angle on one side of the luminous flux entering into the pair of conical prisms are s [], 90-s [], and b [], respectively, +b35 [] is satisfied; and the numeric aperture of the luminous flux entering into the pair of conical prisms is 0.1, and the diameter of the luminous flux entering into the pair of conical prisms is 20 mm.