The present invention relates to an original illuminating apparatus for use in electrophotographic copying machine, original reader of fascimile equipment, etc. The illumination apparatus includes a transparent member for defining a window through which the original is opposed to the optical system for projecting an image of the original on a photosensitive surface. The transparent member is fixed to and supported by a supporting part. The illumination light emitted from a light source is reflected towards the original so as to illuminate it by the reflected light. According to the present invention, the transparent member supporting part and the light reflecting part are formed integrally with a heat conductive member so that the two parts are thermally connected with each other. A heat pipe is provided to deprive the heat conductive member of heat thereby preventing a rise of temperature of the transparent member.