In an image reader, an optical scanning unit reads an original image on an original document sheet. In the optical scanning unit, a lighting device is detachably attached to a housing to emit light toward the original document sheet. The housing houses a reading device to read the light reflected by the original document sheet. A shield member is detachably attached to the housing to shield the reading device from the lighting device to form a space for the reading device inside the housing. A slit is provided in the shield member to guide the light reflected by the original document sheet to the space for the reading device. A second positioner is provided on the shield member to engage a first positioner provided in the housing to position the reading device with respect to the lighting device.