In an image reading device, an opposing member opposes an image reading unit. The opposing member has a first part and a second part. A moving unit moves the image reading unit between first and second read positions. An image value generated by reading an image of the first part at the first read position is different from an image value generated by reading an image of the second part at the second read position. The control device is configured to: set a read position to selected one of the first read position and the second read position; control the moving unit to move the image reading unit to the read position; and determine that an edge of the original sheet reaches the reading region when image values generated by the image reading unit are changed while the conveying unit conveys the original sheet along the conveying path.