A compact illuminating apparatus for viewing films by transmitted light and having uniform light distribution across a diffusing plate, comprising a shallow elongated box-like main body having a base with a bottom having alternately arranged mountains and valleys walls or side parts of the box adapted to hold said diffusing plate; a frame arranged to close the box like base; a reflecting member having reflectance higher than that of said base and arranged on said bottom of said base; and an illuminating member comprising (1) light sources disposed below said diffusing plate, and (2) light-source-control member connected electrically with said light sources for controlling the operation thereof.