The automatic image-property identification is applied for a two-dimensional object on the document table of a scanner. Various informative frames are used for circumscribing the borders of the document sheets. Each frame type is associated with the image properties of a sheet size. The frame types and their associated image properties are stored in a template. When scanning, the program first detects the features of the frame type currently in use. After a frame type is found, the photoelectric converting device can directly read the image properties of the sheet from the template without a prescan.