PROBLEM TO BE SOLVED: To provide an image forming system, which prevents wasteful consumption of electric power due to an initial startup operation for sealing and also controls a decline in productivity of a sealed envelope even when both a plurality of image forming jobs accompanied by sealing operation and at least one image forming job without sealing operation are mixedly included.SOLUTION: The image forming system includes: a paper feeding device 1B for feeding an envelope and a paper sheet to be enclosed in the envelope; an image forming device 1 for forming an image on the paper sheet or envelope; and an enclosing sealing device 4 for enclosing the paper sheet in the envelope and then sealing the envelope, in which the paper sheet is enclosed, by attaching a sealing agent. When both the plurality of image forming jobs accompanied by sealing operation and at least one image forming job without sealing operation are mixedly included, the system divides the jobs into a group of the image forming job accompanied by sealing operation and a group of the image forming job without sealing operation to control an execution order of each image forming job.