An image reading device includes light-emitting diode chips (11A-11C) and a light guide (3) having a main region (3A) and a subsidiary region (3B). The subsidiary region (3B) includes a light incident surface (31) located above the light source (11A-11C) and an inclined surface (32). The light source (11A-11C) is offset in a direction receding from the main region (3A) with respect to the center (C2) as viewed in the primary scanning direction of the light incident surface (31).