1. An image forming apparatus comprising an image reading unit that reads an image, the image forming apparatus comprising: a setting unit constructed to compare a designated size of an original document designated by a user with a detected size of the original document detected by an original document size detecting unit, so as to set a set size of the original document to the larger of the designated size and the detected size for determining an area to be read by the image reading unit; a scaling factor deciding unit constructed to decide a scaling factor of an output image to be output onto recording paper based on the set size of the original document and the size of the recording paper for use in outputting the image read by the image reading unit; a determining unit constructed to determine whether a job-lock function is set to be exercisable or unexercisable; and a scan area deciding unit constructed to decide an area to be scanned on the original document to be read by the image reading unit based on the decision result by the scaling factor deciding unit and the determination result by the determining unit, wherein the scan area deciding unit decides an area on the original document required for outputting the output image onto the recording paper as the area to be scanned when it is determined that the job-lock function is unexercisable and it is decided that the scaling factor of the output image is a scaling factor in an enlargement direction, the scan area deciding unit decides the entire area of the original document as the area to be scanned when it is determined that the job-lock function is unexercisable and it is decided that the scaling factor of the output image is a scaling factor other than the scaling factor in the enlargement direction, the scan area deciding unit decides an area wider than the entire area of the original document as the area to be scanned when it is determined that the job-lock function is exercisable and it is decided that the scaling factor of the output image is the scaling factor other than the scaling factor in the enlargement direction, the scan area deciding unit decides the entire area of the original document as the area to be scanned when it is determined that the job-lock function is exercisable and it is decided that the scaling factor of the output image is the scaling factor in the enlargement direction, and the image reading unit scans an image at the area to be scanned decided by the scan area deciding unit.