An image forming system comprising: an image forming section in which an image is formed on an output medium based on image information; a post processing section in which prescribed post processing is performed on the output medium on which the image is formed by the image forming section; characterised by a booklet division mode judgment section which judges whether a booklet division mode is set or not before the image forming so as to reflect a result of the judgment in the image forming wherein in the booklet division mode, an output medium on which prescribed post processing is to be conducted (Pc) and an output medium on which prescribed post processing is not to be conducted (Pa) coexist among the plural output media on which images are to be formed based on a series of image information, and wherein the output medium on which prescribed post processing is not to be conducted (Pa) has thereon a first image whose orientation is not restricted and the output medium on which prescribed post processing is to be conducted (Pc) has thereon a second image whose orientation is restricted because of the post processing; an image orientation judging section for judging the restricted orientation of the second image if the booklet division mode judgment section judges that the booklet division mode is set; a rotation standard setting section which determines a rotation standard for rotating image information of the first image so that the orientation of the first image in the series of image information is made to agree with the judged orientation of the second image in the same series of image information, and an image processing section which conducts rotation processing on the image information of the first image based on the determined rotation standard. an image forming section in which an image is formed on an output medium based on image information; a post processing section in which prescribed post processing is performed on the output medium on which the image is formed by the image forming section; characterised by a booklet division mode judgment section which judges whether a booklet division mode is set or not before the image forming so as to reflect a result of the judgment in the image forming wherein in the booklet division mode, an output medium on which prescribed post processing is to be conducted (Pc) and an output medium on which prescribed post processing is not to be conducted (Pa) coexist among the plural output media on which images are to be formed based on a series of image information, and wherein the output medium on which prescribed post processing is not to be conducted (Pa) has thereon a first image whose orientation is not restricted and the output medium on which prescribed post processing is to be conducted (Pc) has thereon a second image whose orientation is restricted because of the post processing; an image orientation judging section for judging the restricted orientation of the second image if the booklet division mode judgment section judges that the booklet division mode is set; a rotation standard setting section which determines a rotation standard for rotating image information of the first image so that the orientation of the first image in the series of image information is made to agree with the judged orientation of the second image in the same series of image information, and an image processing section which conducts rotation processing on the image information of the first image based on the determined rotation standard.