An image forming system includes an image forming device, a reading device and a hardware processor. The image forming device includes an image former that forms an image on a sheet. The reading device includes a line sensor and a lens unit. The line sensor reads the sheet with the image formed, thereby obtaining a read image. The lens unit includes a plurality of lenses and forms an image of the sheet on the line sensor. The hardware processor identifies a sheet region of the sheet in the read image, and corrects a magnification of each subregion in a main scanning direction of the sheet region according to a characteristic of a portion in the main scanning direction of the lens unit, the portion having being used in forming an image of the subregion on the line sensor.