roller 2 and the development sleeve 42 respectively of the process cartridge from the electrical circuits provided on the apparatus main assembly side in other words the image forming apparatus becomes ready for an image forming operation a reference numeral 13 designates an exposure window provided in the process cartridge between the cleaning apparatus 5 and the development apparatus 4 the laser beam 33 emitted from the laser scanner unillustrated of the apparatus main assembly enters the process cartridge through this exposure window 13 to scan that is expose the surface of the photosensitive drum 1 also in this embodiment the main assembly of the image forming apparatus is provided with the same charging current detecting means 7 as the one described in embodiment 1 with the structure described above in place the level of the charging current is detected following the timing sequence given in fig4 first the primary bias is turned on and the end and middle portions of the photosensitive drum 1 are exposed by the laser scanner then while these exposed portions are recharged the level of the current slowing into the photosensitive drum 1 from the bias power source 6 is measured by the charging current detecting means 7 to compute the difference in the photosensitive layer thickness of the photosensitive drum 1 between the end and middle portions from which the residual service.