1. An image reading device for determining an image region of a captured image comprising: an image capturing unit that captures a manuscript; a predetermined image pattern, wherein the predetermined image pattern is displayed in a region that is captured by the image capturing unit and provides a background of the manuscript; a region determination unit that determines an image region corresponding to the manuscript, based on an image element corresponding to the image pattern and another image element corresponding to a shadow of the manuscript in an image captured by the image capturing unit; a correction instruction unit that generates correction instruction information based on the image element that corresponds to the image pattern captured by the image capturing unit; a correction unit that performs correction processing on the image captured by the image capturing unit based on the correction instruction information generated by the correction instruction unit; and a lining roller that is mounted in the vicinity of the capturing position of the image capturing unit and rotates according to a conveyance speed of the manuscript, wherein the image pattern is formed on a surface of the lining roller.