An optical system for image projection is disclosed which is easy to be manufactured and designed and capable of projecting a bright image with high contrast. The optical system includes an illumination optical system introducing a luminous flux emitted from a light source to an image-forming element through an optical surface having a light-splitting function, and a projection optical system projecting the luminous flux from the image-forming element through the optical surface onto a projection surface. The illumination optical system includes a conversion system which respectively converts the widths of the luminous flux in first and second cross-sections into widths different from those before entering thereinto. Conversion rates in the first and second cross-sections are different from each other.