An image input device that includes an illumination optical system to illuminate an original with light, a reading unit imaging light transmitted by the original, an original retaining unit retaining the original during a reciprocating motion of the original, and a reciprocating motion guide unit guiding the reciprocating motion of the original retaining unit. The original retaining unit has an aperture portion of a size not covered by the original. Output adjustment of the reading unit is performed in accordance with the light which passes through the portion of the aperture which is not covered by the original. A reciprocating motion guide unit is included to guide the reciprocating motion of the original retaining unit. The original retaining unit has an aperture portion having a first region through which the light of the original is caused to pass and a second region through which light is caused to pass in order for output adjustment of the reading unit. A drive unit causes a reciprocation motion of the original retaining unit in the direction of the optical axis of the light provided by the illumination optical system. An original ejection unit ejects the original and an original holder is slidably retained for insertion and removal in the original retaining unit. The original retaining unit has an aperture portion used by the image input device to perform output adjustment of the reading unit in accordance with light passing through the aperture portion.