1. An image forming apparatus comprising: an image forming device that forms a toner image on a sheet, said image forming device including a photosensitive member, a charging device configured to charge said photosensitive member, an exposure device configured to expose said photosensitive member to light to form an electrostatic image thereon, and a developer that develops the electrostatic image with toner to form the toner image that is transferable to the sheet; a detection device that detects the density of a patch image formed by said image forming device; an input device configured to input by a user one of a plurality of image forming modes including a low resolution mode in which the resolution of the toner image is low and a high resolution mode in which the resolution of the toner image is higher than the resolution of the toner image in the low resolution mode; and real-time gradation correction means for forming patch images in real-time to perform real-time gradation control based on the resolution mode input by the input device in an area corresponding to a sheet interval area on said photosensitive member per a first number of sheets in a case where images are continuously formed onto sheets in a low resolution mode, correcting the gradation of the toner image in said image forming device based on said inputted resolution mode in real time and based on each density of the patch images detected by said detection device, forming patch images in real-time to perform real-time gradation control based on the resolution mode input by the input device per a second number of sheets that is less than the first number of sheets in a case where images are continuously formed onto sheets in a high resolution mode, and correcting the gradation of the toner image in said image forming device in real time based on each density of the patch images detected by said detection device based on the resolution mode input by the input device.