An image forming system of an aspect of the present invention includes: a first image forming portion; a second image forming portion connected in series to the first image forming portion; and an assignment determining portion that determines an assignment of image formation of print data to the first image forming portion and the second image forming portion in accordance with contents of the print data. The first image forming portion forms a first image on the continuous sheet on the basis of the assignment determined by the assignment determining portion, and the second image forming portion forms a second image on a region which is different from a region where the first image is formed on the continuous sheet and which overlaps a non-printable region that is a region where the first image forming portion cannot print, on the basis of the assignment determined by the assignment determining portion.