An image forming apparatus comprising: an image forming device configured to form an image on a recording medium; an image reader configured to read an image from a recording medium; a storage configured to store image data for adjustment of image formation, the image data comprising a cross-hair to be formed by the image forming device in a corner of a recording medium; and an adjuster configured to: read the image data from the storage, form an image for adjustment of image formation on a recording medium with the image forming device according to the image data, read the image for adjustment of image formation from the recording medium with the image reader, and adjust image formation according to read data provided by the image reader, characterized in that: the image data for adjustment further comprising a corner edge detection mark to be formed to detect a folded corner edge of the recording medium; wherein the cross-hair is formed in each corner on a front side of the recording medium, and the corner edge detection mark is formed on a back side of the recording medium at a position where, when the corner of the recording medium is folded such that the cross-hair and the corner edge are positioned in an image read range of the image reader, the folded corner edge of the recording medium is positioned; wherein, when a back side of a recording medium is placed on the image reader, having a corner of the recording medium, which includes an area where the cross-hair is formed on the front side and which is folded to the back side of the recording medium, the adjuster is configured to read the back side with the image reader, detect the cross-hair and the corner edge of the recording medium from read data provided by the image reader, calculate a distance between the cross-hair and the corner edge of the recording medium, and adjust the image formation according to the distance between the cross-hair and the corner edge of the recording medium. an image forming device configured to form an image on a recording medium; an image reader configured to read an image from a recording medium; a storage configured to store image data for adjustment of image formation, the image data comprising a cross-hair to be formed by the image forming device in a corner of a recording medium; and an adjuster configured to: read the image data from the storage, form an image for adjustment of image formation on a recording medium with the image forming device according to the image data, read the image for adjustment of image formation from the recording medium with the image reader, and adjust image formation according to read data provided by the image reader, read the image data from the storage, form an image for adjustment of image formation on a recording medium with the image forming device according to the image data, read the image for adjustment of image formation from the recording medium with the image reader, and adjust image formation according to read data provided by the image reader, the image data for adjustment further comprising a corner edge detection mark to be formed to detect a folded corner edge of the recording medium; wherein the cross-hair is formed in each corner on a front side of the recording medium, and the corner edge detection mark is formed on a back side of the recording medium at a position where, when the corner of the recording medium is folded such that the cross-hair and the corner edge are positioned in an image read range of the image reader, the folded corner edge of the recording medium is positioned; wherein, when a back side of a recording medium is placed on the image reader, having a corner of the recording medium, which includes an area where the cross-hair is formed on the front side and which is folded to the back side of the recording medium, the adjuster is configured to read the back side with the image reader, detect the cross-hair and the corner edge of the recording medium from read data provided by the image reader, calculate a distance between the cross-hair and the corner edge of the recording medium, and adjust the image formation according to the distance between the cross-hair and the corner edge of the recording medium.