An image forming section forms a printed image on paper based on image data. A post-printing processing section performs post-printing processing on the paper. A stabilization section performs stabilization on image data. A sheet interval detection section detects a sheet interval between consecutive sheets being conveyed in the post-printing processing section. A sheet interval determination section determines whether or not the sheet interval exceeds a reference period of time. A cumulative time measuring section measures a cumulative total time of an image formation operation. A time determination section determines whether or not the cumulative total time exceeds a predetermined period of time. A control section controls, upon determination that the cumulative total time exceeds the predetermined period of time and the sheet interval exceeds the reference period of time, the stabilization section to perform the stabilization during the sheet interval exceeding the reference period of time.