An image forming apparatus forms a stamp image with a stamp image developer having a color developing temperature at a color erasing temperature of an image forming developer, and forms a mask image with a mask image developer having a color re-developing temperature at a temperature not lower than a color re-developing temperature of the image forming developer. Accordingly, when a confidential document is color-erased, the stamp image develops color, and when the color-erased confidential document again develops color, it can not be read.