To provide a post-processing system for enabling a sheet prepared in a different printing apparatus to be mixed and undergo post-processing when the post-processing apparatus performs bookbinding finish on sheets with images formed thereon, a collecting means is provided in a post-processing section for performing post-processing on image-formed sheets, and is configured to enable a bunch of sheets to be manually set from the outside, a control section is provided with operating means for outputting a set finish signal such that the sheets have been set manually in the collecting means, and it is configured when a mode selecting means selects a mixing post-processing mode, the control section performs the post-processing on a bunch of sheets by post-processing means using the set finish signal from the operating means.