An image forming apparatus, comprising: a main image forming section for imposing a plurality of jobs, the jobs being imposed with margins therebetween, and thereby generating at least one piece of main image data; an interleaf image forming section for generating an interleaf image depending on the jobs of the main image data and generating interleaf image data including the interleaf image to separate prints printed based on the main image data; wherein in order to generate the interleaf image, the interleaf image forming section is configured to read main image data and to detect positions of the side edge of each job (j1-j8) and the imposed position of each job (j1-j8) of the main image data; establish hypothetical side edges, the hypothetical side edges being side edges that match the detected positions of the side edge of each job (j1-j8) in the imposed position of each job (j1-j8); and to establish color/pattern forming areas (x1 to x8) so as to lie across and over the hypothetical side edges; and to embed color data for a color to be applied to the color/pattern forming areas (x1 to x8)"; the interleaf image forming section further comprising: a data reader for reading two pieces of main image data to be printed before and after printing of an image based on the interleaf image data, based on a print order in which the main image data and the interleaf image data are printed; a job state extractor for extracting the colors of the side edges of the jobs of the two pieces of main image data; and a color/pattern establisher configured to automatically establish, as the color to be applied to the color/pattern forming areas (x1 to x8), a color different from the colors extracted by the job state extractor. a main image forming section for imposing a plurality of jobs, the jobs being imposed with margins therebetween, and thereby generating at least one piece of main image data; an interleaf image forming section for generating an interleaf image depending on the jobs of the main image data and generating interleaf image data including the interleaf image to separate prints printed based on the main image data; wherein in order to generate the interleaf image, the interleaf image forming section is configured to read main image data and to detect positions of the side edge of each job (j1-j8) and the imposed position of each job (j1-j8) of the main image data; establish hypothetical side edges, the hypothetical side edges being side edges that match the detected positions of the side edge of each job (j1-j8) in the imposed position of each job (j1-j8); and to establish color/pattern forming areas (x1 to x8) so as to lie across and over the hypothetical side edges; and to embed color data for a color to be applied to the color/pattern forming areas (x1 to x8)"; the interleaf image forming section further comprising: a data reader for reading two pieces of main image data to be printed before and after printing of an image based on the interleaf image data, based on a print order in which the main image data and the interleaf image data are printed; a job state extractor for extracting the colors of the side edges of the jobs of the two pieces of main image data; and a color/pattern establisher configured to automatically establish, as the color to be applied to the color/pattern forming areas (x1 to x8), a color different from the colors extracted by the job state extractor. a data reader for reading two pieces of main image data to be printed before and after printing of an image based on the interleaf image data, based on a print order in which the main image data and the interleaf image data are printed; a job state extractor for extracting the colors of the side edges of the jobs of the two pieces of main image data; and a color/pattern establisher configured to automatically establish, as the color to be applied to the color/pattern forming areas (x1 to x8), a color different from the colors extracted by the job state extractor.