In an image forming device including an image reading device, dot signals for one scanning line are detected by a CCD sensor. After their A/D conversion and shading-correction, the detected signals are supplied to a comparison processor. In the comparison processor, the dot signals corresponding to the leading and tailing edges of the original are detected from the dot signals for the one scanning line. The rest of the dot signals are judged to be those other than from the original in the selector, and converted into white dot signals. Consequently, the region other than the original is formed in white for each line. Thus, regardless of the original sheet size, or even if the sheet is obliquely placed, the region other than the original can be detected, and the region can be erased by adding a white background.