A reading module includes a light source, an optical system for forming an image of reflection light from a document illuminated by the light source, and a sensor including a plurality of sensor chips to convert the image light formed by the optical system into an electric signal. The optical system includes a mirror array including a plurality of reflecting mirrors connected in the main scanning direction, first aperture stops each of which adjusts light amount of the image light reflected from each of the reflecting mirrors, second aperture stops each extending from the opening edge of the first aperture stop toward the mirror array, first light shielding walls each of which extends from a boundary between the sensor chips toward the first aperture stops, and second light shielding walls extending from both sides in the main scanning direction of the first aperture stop toward the sensor chips.