To provide an illumination device and a projection type image display device that illuminate an area to be illuminated (image formation area) under conditions where speckle noise is less noticeable.An illumination device according to the present invention includes: a light source 11 that emits coherent light; an optical scanning section 15 that scans the coherent light emitted from the light source 11; a lens array 22 including a plurality of element lenses and configured to diverge the light scanned by the optical scanning section; an optical path conversion system 23 configured to control a diverging angle of the diverging light to be emitted from respective points of the lens array 22 and to allow the diverging light whose diverging angle has been controlled to illuminate an area to be illuminated sequentially in an overlapping manner.