To light a document with a constant brightness regardless of the time change of efficiency of an LED by providing, on the circumferential surface of a cylinder, a reflecting mirror having a substantially rectangular opening extending substantially in the central axial direction of the cylinder.

A side mirror has a rectangular slit S extending in the central line C1 direction of a transparent member 121. A part of the light reflected by the inner part of the transparent member 121 gets out of a light conductor 1 through the slit S to form an illuminating light L2, which substantially linearly lights a document 3. In a dimming circuit 2, a CPU compares the brightness designated by an instructing circuit with the brightness of the illuminating light L2. On the basis of the result, the dimming circuit 2 outputs a light emitting current to an LED 112 according to the detection voltage outputted by a photodiode 113. According to this, even if the emission efficiency of the LED 112 is delicately changed according to time, the brightness of the illuminating light L2 can be stabilized constantly.