An illuminating optical system suppresses deterioration in utilization efficiency of illuminating light and generation of fluctuations in light emission without increasing apparatus size. The illuminating optical system includes a plurality of light sources of different types, a light combining optical system that combines light beams output from each of the light sources such that they converge at a single position, and a rod integrator that uniformizes the light intensity distributions of the light beams which are converged at the single position that enter a first end thereof and outputs the light beams, of which the light intensity distribution has been uniformized, from a second end thereof. The light combining optical system includes relay optical component sets corresponding to each of the light sources, and the relay optical component sets have relay magnification ratios different from each other.