An image reading device reads a document using a plurality of reading modes including a sheet-through reading mode for reading a document at a fixed reading position while the document is automatically fed. The image reading device includes a first resolution changer, a second resolution changer, and a resolution setting unit. The first resolution changer changes one of a linear velocity at which the document is fed and a traveling speed of a reading carriage to change a resolution at which the document is read in a sub-scanning direction. The second resolution changer electrically interpolates image data output from the first resolution changer to change a resolution of the image data. The resolution setting unit provides a plurality of combinations of resolution change rates between the resolution changers as resolution change modes and sets the resolution changers to one of the combinations of resolution change rates.