1. An image reading device comprising: a light source configured to sequentially radiate different colors of light to a document; an imaging element configured to receive reflected light from a linear reading region, in the document, extending in a main scanning direction, and sequentially output image data of respective colors; a scanner configured to move the reading region in the document in a sub-scanning direction; an estimated image data calculator configured to calculate, as estimated image data, for each of the image data of respective colors outputted from the imaging element, on the basis of a pixel value of a first pixel on one of two lines adjacent to each other in the image data, and a pixel value of a second pixel on the other of the two lines and adjacent to the first pixel, image data of a non-reading region between a reading region in the document corresponding to the one of the two lines and a reading region in the document corresponding to the other of the two lines; and a pixel value correcting unit configured to correct at least one of the pixel value of the first pixel and the pixel value of the second pixel on the basis of the estimated image data.