An image-reading device includes an image sensor, a clock generator, a resolution setting unit, and a resolution switching unit. The image sensor has a plurality of photodetectors arranged linearly. The clock generator generates clocks having a clock speed. The resolution setting unit sets a resolution of the image sensor. The resolution switching unit switches the resolution set by the resolution setting unit when one of the plurality of photodetectors has read a border pixel between a pixel segment inside the original document and a pixel segment outside the original document while reading one line of the original document.