A sampling clock generator circuit includes a reference clock generator, a sampling hold circuit, a sampling clock generator to delay an output clock signal from the reference clock generator by a predetermined delay amount to generate and supply a sampling clock signal to the sampling hold circuit, a phase determining element to compare phases of a drive clock signal for an image reading unit and the sampling clock signal to output a result of the phase comparison, the drive clock signal generated according to the output clock signal of the reference clock generator, and a controller to adjust the delay amount of the sampling clock generator on the basis of the result of the phase comparison so that a phase difference between the drive clock signal and the sampling clock signal becomes zero.