An image reading unit includes an image reading section that reads an image of a document in a one-dimensional direction by one-dimensionally aligned imaging elements, a light source that irradiates an image reading region by the image reading section with light, and a rotation head that holds the image reading section and the light source in a manner that the image reading section and the light source are rotatable relative to the document about a rotation axis which is parallel to an alignment direction of the imaging elements.