To perform square back processing without causing a center folding mark on the portion of a spine cover of a booklet.

A post-processing apparatus 300 includes a folding unit 320 configured to nip a sheet using two rollers to form a fold line for saddle stitching and a control unit to control the nipping amount in the folding unit. A post-processing method applies post-processing to the sheet with an image formed thereon using the apparatus. When carrying out square back processing to flatten the spine folded portion of a sheet bundle where a fold line for saddle stitching is formed, a control unit allows the nipping amount in the folding unit to be smaller than that when the square back processing is not carried out.