An image reading device 1 includes an output acquiring unit 21 acquiring outputs from a plurality of light receiving elements 17 arranged to form an image reading surface for reading an image of an original, a difference detecting unit 22 detecting edges in the original or an end portion of the original on the basis of the outputs from at least some of the light receiving elements 17 in the outputs acquired by the output acquiring unit 21, and an image information generating unit 23 generating the image information of the original by use of the outputs from the plurality of light receiving elements 17 when the difference detecting unit 22 detects the edges or the end portion of the original.