To provide an image forming device which can efficiently prevent the leakage of instruction information to properly instruct post-processing to a bunch of sheets, and a post-processing instruction method and a post-processing instruction program.

In the image forming device, a print image is generated based on a print job, and printed on a plurality of sheets, and subjected to post-processing with a post-processing device to be completed as printed matter and is discharged. The image forming device includes a post-processing instruction part which, when generating the print image, adds indication information regarding the post-processing in a predetermined region of a group of sheets which constitute at least part of a bunch of sheets so that the indication information is made to be readable in a first mode before the post-processing and made to be unreadable in a second mode after the post-processing.