a light source which irradiates a reading object with light;

a photoelectric conversion element including a plurality of color sensors upon which a reflected light from the reading object is incident and which are long in a main scanning line direction;

a lens disposed on an optical path of the light guided into the photoelectric conversion element from the light source;

a white reference chart comprising a first concentration;

a line reference chart comprising the first concentration and a second concentration corresponding to a darkest portion of an image;

a balance judgment mechanism which judges whether or not an amplitude of a line signal based on the reflected light from the line reference chart satisfies a color balance condition, by executing an MTF balance mode, based on the first concentration corresponding to a brightest portion of the image based on a reflected light from the white reference chart and the second concentration;

a moving force generation mechanism which applies a predetermined moving force in a case where the light signal does not satisfy the balance condition; and

a moving mechanism to move the lens and the photoelectric conversion element and to change an optical path length between the light source and the lens while securing a certain optical path length between the lens and the photoelectric conversion element, when the predetermined moving force is applied to the moving mechanism from the moving force generation mechanism,

wherein the line reference chart includes a first chart, and first and second line patterns, and is disposed in a middle portion in which an optical axis of light to be irradiated is positioned and whose image height is 0%, or

the line reference chart includes a second chart, and first and second line patterns, and the disposing position corresponds to an image height 70% or more using the middle portion as a reference.