An image reading unit according to the present invention includes a lamp to incorporate a change in temperature conditions to the emitting light amount, the lamp capable of illuminating an reading object with a light of a predetermined intensity, an image signal generation unit which generates an image signal corresponding to the reflected light generated when a reading object is illuminated, and a cooling unit which cools a predetermined range of the lamp, so that the intensity change in the light radiated from the lamp of the illumination unit, in the length direction of the illumination unit can be suppressed.