1. An image reading device comprising: a conveying unit defining a conveyance path and configured to convey a sheet along the conveyance path in a conveying direction, the conveyance path including a reading position; an image reading unit configured to execute a reading process; and a control unit configured to: control the image reading unit to start executing the reading process before the sheet conveyed by the conveying unit reaches the reading position and to subsequently and repeatedly execute the reading process, the image reading unit acquiring a plurality of partial images successively, each of the plurality of partial images being acquired each time the image reading unit executes the reading process; determine, each time one partial image is acquired, a number of pixels awarded to the one partial image; determine, each time one partial image is acquired, an intensity awarded to the one partial image; judge, each time one partial image is acquired, whether or not two successively-acquired partial images satisfy at least one of: a condition (A) that the determined number of pixels is changed between the two successively-acquired partial images; and a condition (B) that the determined intensity is changed between the two successively-acquired partial images; and decide that at least one of the two successively-acquired partial images corresponds to an end of the sheet in the conveying direction when the two successively-acquired partial images satisfy at least one of the conditions (A) and (B).