An image forming apparatus including hardware resources to use for an image forming process, at least one program for conducting a process concerning an image formation, and a hard disk for recording information including an image, said image forming apparatus including: an erase information generating part generating erase information for erasing the information recorded in the hard disk; and an erase information overwriting part overwriting the erase information on the information at a sector unit or a cluster unit; characterized in that the image forming apparatus is configured to overwrite a sector unit or cluster unit only if the hard disk is not used by another process; and the erase information generating part and the erase information overwriting part are adapted to perform the steps of a) generating a random number, b) determining whether or not a calculation based on the random number is to be conducted, wherein the calculation is mapping of the random number into a new random number, c) performing the calculation if it is to be conducted, d) overwriting the sector unit or cluster unit with the random number, e) determining if all sector units or cluster units have been overwritten, f) if not all sector units or cluster units have been overwritten, determining whether or not a random number is to be generated for each sector unit or cluster unit, and proceeding to step a) if this is the case and to step d) if this is not the case, g) if all sector units or cluster units have been overwritten, determining whether or not the overwriting process has been completed for a second time and proceeding to step a) if this is not the case, h) if the overwriting process has been completed for the second time, checking whether or not an operator sets a value to overwrite, i) if the operator sets a value to overwrite, overwriting all sector units or cluster units with the value to overwrite, and j) if the operator does not set a value to overwrite, overwriting all sector units or cluster units with zero. an erase information generating part generating erase information for erasing the information recorded in the hard disk; and an erase information overwriting part overwriting the erase information on the information at a sector unit or a cluster unit; characterized in that the image forming apparatus is configured to overwrite a sector unit or cluster unit only if the hard disk is not used by another process; and the erase information generating part and the erase information overwriting part are adapted to perform the steps of a) generating a random number, b) determining whether or not a calculation based on the random number is to be conducted, wherein the calculation is mapping of the random number into a new random number, c) performing the calculation if it is to be conducted, d) overwriting the sector unit or cluster unit with the random number, e) determining if all sector units or cluster units have been overwritten, f) if not all sector units or cluster units have been overwritten, determining whether or not a random number is to be generated for each sector unit or cluster unit, and proceeding to step a) if this is the case and to step d) if this is not the case, g) if all sector units or cluster units have been overwritten, determining whether or not the overwriting process has been completed for a second time and proceeding to step a) if this is not the case, h) if the overwriting process has been completed for the second time, checking whether or not an operator sets a value to overwrite, i) if the operator sets a value to overwrite, overwriting all sector units or cluster units with the value to overwrite, and j) if the operator does not set a value to overwrite, overwriting all sector units or cluster units with zero. a) generating a random number, b) determining whether or not a calculation based on the random number is to be conducted, wherein the calculation is mapping of the random number into a new random number, c) performing the calculation if it is to be conducted, d) overwriting the sector unit or cluster unit with the random number, e) determining if all sector units or cluster units have been overwritten, f) if not all sector units or cluster units have been overwritten, determining whether or not a random number is to be generated for each sector unit or cluster unit, and proceeding to step a) if this is the case and to step d) if this is not the case, g) if all sector units or cluster units have been overwritten, determining whether or not the overwriting process has been completed for a second time and proceeding to step a) if this is not the case, h) if the overwriting process has been completed for the second time, checking whether or not an operator sets a value to overwrite, i) if the operator sets a value to overwrite, overwriting all sector units or cluster units with the value to overwrite, and j) if the operator does not set a value to overwrite, overwriting all sector units or cluster units with zero.