An image reading device includes a platen on which an original is to be placed, an illuminating unit configured to illuminate the original, a linear sensor configured to receive light from the original, an optical unit configured to guide the light from the original to the linear sensor, and an aligning portion configured to determine a position of the original in a long-side direction of the linear sensor. The illuminating unit includes a light-guiding member extending in the long-side direction of the linear sensor, and a light source provided only at one long-side end of the light-guiding member. The aligning portion is provided on a side opposite to the light source in the long-side direction of the light-guiding member.