An image reading apparatus includes a DC/DC power supply which supplies power to at least part of the apparatus, a photodetector array including two-dimensionally arranged photoelectric conversion elements, and a line selector and reading circuit which read signals from the photoelectric conversion elements for each row in the photodetector array as a unit. A line driving signal in the line selector is set to be synchronized with a reference clock for defining the oscillation frequency of the DC/DC power supply and have a period of an integer multiple of the reference clock. A sample/hold signal for determining the timing at which a signal read from the photodetector array is sampled/held is output at a predetermined phase timing of each period of the line driving signal.