The illumination optical system includes a beam shaper which converts an intensity distribution of a laser beam in each of a short axis direction and a long axis direction, which is a Gaussian distribution, into an intensity distribution of a parallel beam on a modulation surface of the optical modulator in each of the short axis direction and the long axis direction, which is a top hat distribution. The modulation surface and an irradiated surface are optically conjugated with respect to the long axis direction by a third lens and a fourth lens. Further, the modulation surface and a front focus position of the fourth lens are optically conjugated with respect to the short axis direction by a first lens, a second lens, and the third lens. The fourth lens condenses a beam having a top hat distribution at the front focus position onto the irradiated surface.