An image sensor unit includes: a light source that illuminates a document; a rod-shaped light guide including a reflection surface that reflects light from the light source and an emission surface provided on a side opposite to the reflection surface that emits light reflected by the reflection surface to the document; a thermal conductor provided so as to cover the reflection surface of the light guide; an image forming element that forms an image of reflected light from the document on a photoelectric conversion element; a sensor substrate on which the photoelectric conversion element is mounted; and a frame including a locator for mounting the light guide.