1. An image reading apparatus that converts light reflected from an original into an analog image signal by a photoelectric conversion element, that converts the obtained analog image signal, to which analog signal processing is applied, into a digital image signal by an analog-digital converter, and that outputs the digital image signal, the image reading apparatus comprising: a driving unit that drives the photoelectric conversion element by a clock whose frequency is modulated; a signal generating unit that generates an analog signal corresponding to a frequency change of the clock whose frequency is modulated; an amplification inversion unit that is capable of amplifying an alternating current component of the analog signal with an amplification factor by a setting value that is set from outside, and that is capable of switching the alternating current component between inversion and non-inversion; a signal superimposing unit that superimposes the analog image signal on an analog signal that is output from the amplification inversion unit; and a controlling unit that converts analog image signal that is superimposed by the signal superimposing unit into a digital image signal by using the analog-digital converter, and that controls the amplification factor and the inversion and non-inversion of the amplification inversion unit based on a feature amount that appears on the digital image signal, wherein the controlling unit uses a detection value of a peak-bottom difference of the digital image signal in a predetermined image area as the feature amount.