A projection type display apparatus, wherein a common case can be used for housing an illumination optical system even when a shape of an image display element and an optical path length in a prism are different, is provided: wherein a light source for emitting an illumination light, an image display device, an illumination optical system for irradiating the illumination light from the light source to the image display device, and a projection optical system for projecting an image formed by said image display device are provided; the illumination optical system comprises a first fly-eye lens and a second fly-eye lens arranged at each other's focal points, a first lens group and a second lens group arranged over a mirror for bending an optical path; the first and second lens groups have a same focal length but a different back focus; and predetermined condition formulas are satisfied.