An illuminating device for linearly illuminating a flat object, in particular a bank note, includes at least one or a plurality of linearly disposed light sources, a mirror arrangement serving as a reflector that extends in parallel to the linearly disposed light sources, with an optical axis perpendicular thereto. The mirror arrangement includes plane mirrors, with at least one first mirror having a structure, which images the light source to form at least one linear image in parallel to the mirror arrangement. At least one second mirror images the at least one linear image to form a linear illumination in parallel to the mirror arrangement on the optical axis or in the proximity of the optical axis.