Two illuminating units 2L and 2R, each having a light guide provided at its one end, with a light-emitting source, are provided. Each illuminating unit is arranged in such a manner that light emitted from its emission plane irradiates the same area of a document-reading plane and the light-emitting source is situated on the opposite side in a main-scanning direction (i.e. the longitudinal direction). By alternately arranging two illuminating units, it is possible to offset characteristics of each illuminating unit so that uniformity and equalization of light intensity distribution can be improved. Since each-illuminating unit is arranged so that its emission plane is symmetric relative to a plane at right angles to the document-reading surface, it is possible to reduce shading that is produced on a document surface when the document surface is not flat.