An overlap determining unit determines whether or not a stamp character string is readable using image data of a stamp image and image data of a scan image read by a scanner unit when the stamp image and the scan image area overlapped. A stamp image generating unit generates a correction stamp image obtained by correcting the stamp image to improve readability of the stamp characters when it is determined that the stamp characters are not readable. A synthesis image generating unit generates a synthesis image in which the correction stamp image and the scan image are overlapped. A printer unit forms the synthesis image on a printing medium.