An image sensor includes: a light transmissive member extending in a direction orthogonal to a direction of conveying the document; a reference portion provided outside the conveying region where a document is conveyed with respect to the orthogonal direction and having a transmittance lower than a transmittance of in the conveying region of the light transmissive member; and a light source unit. The image sensor further includes: a lens unit converging the light transmitting through the document and the reference portion; a light receiving unit that receives the light transmitted through the lens unit and outputs an electric signal; an amplifying unit that amplifies the electric signal of the document corresponding to the conveying region and outputs an image signal of the document; and a controlling unit configured to control an amplification factor of the amplifying unit based on the electric signal corresponding to the reference portion.
