1. An image-processing apparatus comprising: a memory that stores standard sheet size information and size tolerance information; and a processor coupled to the memory, wherein the processor executes a process comprising: clipping document image data of a document image by deskew processing and crop processing on read image data of read images read by an image-reading apparatus; determining whether the document image corresponds to a standard sheet, based on the standard sheet size information and the size tolerance information, and acquiring scan information of the read images read by the image-reading apparatus when determining that the document image corresponds to the standard sheet, wherein the memory stores temporal statistical information of the read images read by the image-reading apparatus, and the process further comprises: calculating an aspect ratio of the document image included in the read images read by the image-reading apparatus; generating temporal change data of a time to the aspect ratio for each standard sheet based on the statistical information and the aspect ratio; and outputting alarm information for notifying an abnormal state in a roller of the image-reading apparatus when determining that the temporal change data fail to satisfy a threshold.