Abstract: An illumination optical system configured to illuminate a mask on which a predetermined pattern is formed, includes a plurality of light sources configured to emit pulse lights, an optical system including a division part configured to divide the pulse lights emitted from the plurality of light sources into first pulse light and second pulse light, a delay optical system configured to guide the second pulse light to a second optical path longer than a first optical path through which the first pulse light passes, and a synthesis/division part configured to synthesize the first pulse light and the second pulse light passing through the delay optical system and divide and emit the synthesized pulse light, and an illumination system configured to guide the pulse lights emitted from the optical system to the mask and illuminate the mask.