To provide an image reader where affinity is given between actual temperature circumstances of document platen and comparatively small number of temperature sensors of the document platen by making temperature gradient on the document platen appropriate.

A scanner part constituted of a lamp unit 6 irradiating an original placed on the document platen 8 and one part of a mirror unit 7 is moved from a 1st position where reading an original image is started to a 2nd position where the reading the document image is finished. Every time the reading the document image is finished, an image read part is moved from the 2nd position to the 1st position. After copying operation (original image reading operation) is finished, the scanner part is moved by a specified distance from the 1st position in accordance with the temperature detected by document platen temperature sensor 13 detecting the temperature of the document platen.