Abstract: An illumination apparatus is disclosed which is suitable for use in a projection/exposure system for projecting an image of a circuit pattern of a reticle on a semiconductor wafer through a projection lens. The illumination apparatus includes an excimer laser for emitting a pulsed laser beam, an optical system for illuminating the reticle with a plurality of laser pulses emitted from the excimer laser so that laser pulses having passed through the reticle impinge on the semiconductor wafer in different directions through the projection lens, and a light intensity control device for controlling the light intensity of each of the laser pulses so that the laser pulses equally contribute to the reaction of a light sensitive material which is provided on the semiconductor wafer, with light.