An image forming apparatus includes one or more processors and one or more memories storing program instructions, which, when being executed by the one or more processors, causes the one or more processors to generate a print image to be printed on a sheet of paper and a position correcting mark for front and back side registration having a brightness lower than a minimum brightness of the print image; control printing of the print image and the position correcting mark on a first side of the sheet of paper; control reading the print image and the position correcting mark from the first side and control generating of read image data; and detect, from the read image data, the position correcting mark on the first side using a brightness difference.