An image forming apparatus for performing a ring binding job wherein a sheaf of sheets is prepared by forming an image first on the n-th sheet (Pn) as a rear cover positioned at the bottom portion, forming an image second on the first sheet as a front cover, forming an image third on a second sheet and stacking it and forming an image lastly on an n-lth sheet (Pn-1) and stacking it, the prepared sheaf of sheets is combined by providing a ring binder to the sheaf of sheets, and said ring binder combining the sheaf of sheets is covered with the n-th sheet of the sheaf of sheets having said ring binder by rotating the n-th sheet from the bottom position to the top position, wherein the image forming apparatus comprises, an image processor configured to process image data of a page-description language in order of a first sheet through the n-th sheet (Pn) and to convert the processed image data into scanning line type image data in the ring binding job; an image forming section configured to receive the scanning line type image data in sequence from said image processor and to form an image; and a controller configured to determine whether the image process of converting the image data of the n-th sheet (Pn) into the scanning line type image data is required or not, wherein when the image process for the image data of the n-th sheet (Pn) is not required, said controller is configured to control said image processor so as to notify said image forming section that the image data of the n-th sheet (Pn) is out of the target of the image process and to apply the scanning line type image data to said image forming section in sequence, the scanning line type image data being derived by executing the image process in order of the first through n-lth sheets; and characterized in that when said controller determines whether the image process of converting the image data of the n-th sheet (Pn) into the scanning line type image data is required or not, said controller is configured to detect whether an image forming job with the image data saved in said image forming section is designated for the n-th sheet (Pn) or not and to determine that the image process for the image data of the n-th sheet (Pn) is not required when the image forming job with the image data saved in said image forming section is designated. wherein the image forming apparatus comprises, an image processor configured to process image data of a page-description language in order of a first sheet through the n-th sheet (Pn) and to convert the processed image data into scanning line type image data in the ring binding job; an image forming section configured to receive the scanning line type image data in sequence from said image processor and to form an image; and a controller configured to determine whether the image process of converting the image data of the n-th sheet (Pn) into the scanning line type image data is required or not, wherein when the image process for the image data of the n-th sheet (Pn) is not required, said controller is configured to control said image processor so as to notify said image forming section that the image data of the n-th sheet (Pn) is out of the target of the image process and to apply the scanning line type image data to said image forming section in sequence, the scanning line type image data being derived by executing the image process in order of the first through n-lth sheets; and characterized in that when said controller determines whether the image process of converting the image data of the n-th sheet (Pn) into the scanning line type image data is required or not, said controller is configured to detect whether an image forming job with the image data saved in said image forming section is designated for the n-th sheet (Pn) or not and to determine that the image process for the image data of the n-th sheet (Pn) is not required when the image forming job with the image data saved in said image forming section is designated.