1. An image-reading device comprising: a reading unit configured to read an original document as image data; a displaying unit configured to display the image data read by the reading unit; a memory storing a page number table; a processor configured to: set a recognition target region as part of the displayed image data; analyze the image data and recognize a page number in the analyzed image data of the recognition target region; determine a number of pages actually read by the reading unit; register the recognized page number and the number of pages in the page number table; determine a particular value by implementing a calculation in accordance with a formula (maximum page number−minimum page number+1), the maximum page number being the greatest page number among the recognized page numbers registered in the page number table, and the minimum page number being the least page number among the recognized page numbers registered in the page number table; determine that a page reading error has occurred when the particular value is different from the number of pages registered in the page number table; determine that the page reading error has occurred when the recognized page number is already stored in the page number table; when no page number is recognized in the image data, set a provisional page number corresponding to the image data in which no page number is recognized, and register the provisional page number in the page number table; and when a new page number is recognized in new image data that is obtained by reading a new original document and when the new page number is equivalent to the provisional page number, change the provisional page number in the page number table to a number different from both the page number and the new page number in the page number table.