PURPOSE: To prevent the optical reading positions of even originals with various kinds of thickness from being fluctuated.

CONSTITUTION: A reading mechanism in which an automatic paper feed mechanism 1 is provided and which performs the white reference decision of the original with a white sheet 5 is comprised in such a way that a tensile force is applied to the original between the contact plane of the original with a full width roller with stage for original carrying and the contact plane with a roller in the automatic paper feed mechanism 1 and the original can be prevented from floating over the reading position by employing such constitution that a floating prevention guide 8 for original is provided, and the movable guide 9 of the white sheet moving in an original floating direction is unified with the white sheet 5 located on the reading plane of a reading device 2 on which the original is adhered closely and a prevention spring 6 which prevents the white sheet from floating functions in a direction to negate a force in the original floating direction when it is applied to the movable guide 9 of the white sheet.