In one embodiment of an image forming apparatus according to the invention, an overwrite data determination device for determining overwrite data so that a hard disk containing information on image forming jobs is overwritten with the overwrite data. An overwrite block size determination device for determining size of an equal-size block so that a disk memory area is divided into equal-size blocks to be overwritten with the overwrite data. The size of the equal-size block is larger than that of the overwrite data. Using the overwrite settings specified by the overwrite data determination device and the overwrite block-size determination device, a data erase control device overwrites the hard disk with meaningless data in order to erase data stored in the hard disk.