An image reading device includes a light source member having a substrate extending in a predetermined main scanning direction, and light source portions releasing light and supported by the substrate in the main scanning direction; a light guide member that has an input section disposed facing the light source portions and receiving the light, and an output section outputting the light, and that guides the light toward a predetermined radiation position; a support member supporting the light source member and the light guide member and having higher rigidity than the light guide member and the substrate; a first positioning section positioning the light guide member in the main scanning direction and a sub scanning direction; a second positioning section positioning the light guide member in the sub scanning direction; and a reader member receiving the light from the radiation position so as to read an image thereof.