An illuminating optical system for use in a projection exposure device with high utilization efficiency of the light from the light source and capable of easily realizing with simple construction is disclosed. The system comprises a light source, a light flux separating optical system for separating a light flux from the light source, a condenser optical system for leading the light flux separated by the light flux separating optical system on a reticle, and a diffraction optical unit included in the light flux separating optical system and having a linear grating pattern for separating the light flux from the light source into four.