An illumination apparatus for using a beam emitted from a
light source to illuminate an object area includes a condensing optical
system for condensing the beam emitted from the
light source, and a reflection type optical
integrator located between the condensing optical
system and the object area, the optical
integrator having an m-gonal (where m is an even number) sectional shape, forming multiple pairs of opposing reflection surfaces, and reflecting the beam between the multiple pairs of opposing reflection surfaces, wherein a length L in an axial direction of the reflection type optical
integrator meets the following equations: R=Φ / [tan {sin−1(sin θ / n)}]; and C×(A−0.1)×R≦L≦C×(A+0.1) X R, where n is a
refractive index of a medium between the reflection surfaces, Φ is a distance between the reflection surfaces, θ is an angle of
divergence when the beam emitted from the condensing optical
system enters the reflection type optical integrator, C is a constant determined in relation to the sectional shape of the reflection type optical integrator, and A is a natural number.