An image reading optical system includes an image reading unit including plural reading elements that are arranged in a first direction, a first reflective optical system configured to reflect reflection light which is irradiation light emitted from a light source and reflected on an object to be read, a second reflective optical system configured to condense the reflection light reflected on the first reflective optical system to the image reading unit, and an aperture stop including a plate shaped member formed with an aperture configured to regulate the reflection light reflected on the first reflective optical system in the first direction, the plate shaped member including at least one plate shaped piece bent at a preset angle with respect to the aperture along a tangent line of the aperture in the first direction.