1. An image reading device comprising: a document tray on which documents having images recorded thereon are placed in a stacked manner, the document tray allowing a mixture of the documents having different sizes to be placed thereon; a document transport mechanism that sequentially feeds and transports the documents placed on the document tray in a one-by-one manner; an image data generator that extends in a width direction intersecting a moving direction of each document transported by the document transport mechanism and that has an image reading sensor that repeatedly reads, in the width direction, the image from each document fed and transported from the document tray, wherein the image data generator generates image data expressing an image within a target reading region of a currently fed document from the document transport mechanism by permitting the image reading sensor to perform a reading process to a region protruding from the target reading region so as to cause the image reading sensor to perform the reading process over a reading region that does not cause unread parts within the target reading region; an image memory that stores the image data generated by the image data generator and expressing the image within the target reading region; and an image extractor that extracts the image data expressing the image within the target reading region from the image data stored in the image memory, wherein a leading edge of the document reaches a reading position before the target reading region is determined.