An image reading apparatus include a light source; an image sensor configured to output an image signal of an object illuminated by the light source; a first calculating unit configured to calculate a third light amount based on a difference between a first image signal output from the image sensor when the light source illuminates a reference member as the object at a first light quantity and a second image signal output from the image sensor when the light source illuminates the reference member at a second light quantity and a difference between the first light quantity and the second light quantity; and a light quantity setting unit configured to set a light quantity at which the light source illuminates a document as the object to the third light quantity.