An image reading apparatus that reads out an original document image synchronously with a frequency dispersion clock gained by modulating the frequency of a reference clock so as to be changed cyclically at a predetermined modulation period, comprising: a first signal generating section that generates a first signal which turns into assert state at a predetermined period; a second signal generating section that generates a second signal which turns into assert state at a period synchronous with the modulation period of the frequency dispersion clock; and a line scanning section that moves a scanning position on the original document image in an auxiliary scanning direction and scans the original document image in a main scanning direction perpendicular to the auxiliary scanning direction each time when both the first signal and the second signal turn to assert state and thereby reads the original document image and outputs an analog image signal.