An image reading device comprising: an image reading unit for reading a document to generate image data; and; a document feeder unit for feeding set document sheets one by one to a read position of the image reading unit; a setting input unit for accepting a document read instruction in a mixed setting mode in which a plurality of document sheets having different sizes are set on the document feeder unit and are read; a size determination unit for determining a size of each page of the document read by the image reading unit; a blank sheet detection unit configured to recognize, based on the document size determined by the size determination unit, a main scanning width and a sub-scanning width of a blank sheet detection area in image data in which to detect a blank sheet out of the image data, to perform, when the setting input unit has accepted a document read instruction in the mixed setting mode, starting from before reading of one page of the document is completed, a blank sheet detection process to detect whether or not the document sheet is a blank sheet for a plurality of main scanning widths in the blank sheet detection area corresponding to a plurality of document sizes corresponding to the image data by a unit of a plurality of lines in a main scanning direction, and to select, after the reading of one page of the document is completed, out of results of the blank sheet detection process for the plurality of main scanning widths, a result of the blank sheet detection process for a main scanning width corresponding to the document size determined by the size determination unit and then detect, based on a result of the blank sheet detection process within a range of the sub-scanning width corresponding to the document size determined by the size determination unit, whether or not the document is a blank sheet; and a blank sheet deletion unit for deleting image data of the document sheet whose blank sheet detection area is detected as blank, so that the image data is not used for a job. an image reading unit for reading a document to generate image data; and; a document feeder unit for feeding set document sheets one by one to a read position of the image reading unit; a setting input unit for accepting a document read instruction in a mixed setting mode in which a plurality of document sheets having different sizes are set on the document feeder unit and are read; a size determination unit for determining a size of each page of the document read by the image reading unit; a blank sheet detection unit configured to recognize, based on the document size determined by the size determination unit, a main scanning width and a sub-scanning width of a blank sheet detection area in image data in which to detect a blank sheet out of the image data, to perform, when the setting input unit has accepted a document read instruction in the mixed setting mode, starting from before reading of one page of the document is completed, a blank sheet detection process to detect whether or not the document sheet is a blank sheet for a plurality of main scanning widths in the blank sheet detection area corresponding to a plurality of document sizes corresponding to the image data by a unit of a plurality of lines in a main scanning direction, and to select, after the reading of one page of the document is completed, out of results of the blank sheet detection process for the plurality of main scanning widths, a result of the blank sheet detection process for a main scanning width corresponding to the document size determined by the size determination unit and then detect, based on a result of the blank sheet detection process within a range of the sub-scanning width corresponding to the document size determined by the size determination unit, whether or not the document is a blank sheet; and a blank sheet deletion unit for deleting image data of the document sheet whose blank sheet detection area is detected as blank, so that the image data is not used for a job.