In a contact type image sensor including an illumination section including a light source and light guide to illuminate a document, an image sensing element for converting an optical image of the document into an electrical signal, a cylindrical rod lens for forming an optical image on the image sensing element, and a frame for integrally holding the illumination section, image sensing element, and cylindrical rod lens, the light guide has undergone antireflection treatment on a surface thereof on the document side.