An image reading device includes a light source, a photoelectric conversion portion, a first radiation control portion, and a first image generating portion. The light source radiates lights of R (red), G (green), and B (blue). The photoelectric conversion portion receives light reflected from a document and outputs an electric signal corresponding to a light reception amount. The first radiation control portion sequentially radiates two color lights of G and R or B from the light source to each line along a main scanning direction on the document while alternately switching the lights of R and B on a line-by-line basis. The first image generating portion generates color image data for one line, based on at least three electric signals corresponding to lights of R, G, and B among the electric signals corresponding to respective color lights radiated to at least adjacent two lines by the first radiation control portion.