A reading module has a light source, an optical system, and a sensor. The optical system images, as image light, reflected light of light radiated from the light source to a document. In the sensor, the image light imaged by the optical system is converted into an electrical signal. The optical system has a mirror array and an aperture stop portion. In the mirror array, reflection mirrors are coupled together in an array in the main scanning direction. The aperture stop portion has a first aperture adjusting the amount of the image light reflected from a reflection mirror and a second aperture shielding stray light entering the first aperture from an adjacent reflection mirror. Between the first and second apertures, a reflection reduction mechanism is provided that reduces reflection, toward the first aperture, of light other than the image light traveling from the second to first aperture.