Provided is an image reader including: a first reading device for reading an image on one side of a document moving on a transparent glass plate; a second reading device as a movable reading device, for reading one of an image on another side of the document moving on the transparent glass plate and an image on another side of the document remaining stationary on the transparent glass plate; and a show-through preventing member for preventing show-through of the document at a first position opposed to the first reading device, characterized in that the show-through preventing member is provided movably from the first position opposed to the first reading device to a second position in which reading of an image by the second reading device is prevented from being hindered.