The image reading apparatus includes an LED, a light guiding body, and a photoelectric conversion element. The light guiding body has a plurality of prisms for diffusing light. The plurality of prisms includes: at least one first prism that is disposed on an incident surface side in the main scanning direction; and at least one second prism that is disposed on an opposite end side to the incident surface side in the main scanning direction. In the first prism, an angle of a prism incident plane with respect to the main scanning direction is greater than an angle of a prism anti-incident plane with respect to the main scanning direction. In the second prism, an angle of a prism incident plane with respect to the main scanning direction is smaller than an angle of a prism anti-incident plane with respect to the main scanning direction.