PURPOSE: To prevent a leak of secret information from a recording medium generating abnormality and to eliminate the necessity for strictly controlling said recording medium by providing a recording means for recording a pattern image for making recording data undiscernible on the recording medium generating abnormality in image formation on the basis of the detection result of a detection means.

CONSTITUTION: When the abnormality in image formation on a recording medium is detected by a detection means 3, the respective conditions (potential, exposure or the like) of an image forming mechanism and a transfer charger 5 are appropriately changed over to form solid black data making printing data undiscernible on the recording medium generating abnormality. Subsequently, an order is issued to a separation mechanism 7 and the recording medium generating abnormality is transferred to the second paper loading part 9 different from the first paper loading part 8 receiving a normal one to be discharged. As a result, since recording data is undiscernible from the recording medium generating abnormality, a leak of secret information can be prevented and the complicatedness for keeping secrets with respect to the recording medium can be eliminated.