Sheet processing apparatus that properly performs fold line processing, and image forming system

An image forming system comprising:

• an image forming apparatus configured to form an image on a sheet;
• a stacking unit configured to have sheets stacked thereon, which are sequentially received from the image forming apparatus, as a sheet bundle;
• a folding unit configured to fold the sheet bundle stacked on the stacking unit at the center of the sheet bundle in a conveying direction of the sheets;
• a conveying unit configured to convey the sheet bundle folded at the center by the folding unit to a processing position;
• a press unit configured to move along a fold line portion of the sheet bundle conveyed to the processing position by the conveying unit, while pressing the fold line portion from a direction orthogonal to a thickness direction of the sheet bundle, to flatten the fold line portion;
• a first acquisition unit configured to acquire a first pressing strength, which is applied to the fold line portion at a time of flattening the fold line portion designated by a user;
• a second acquisition unit configured to acquire information on the sheet bundle to be processed by the press unit, and acquire a second pressing strength based on the acquired information;
• and
• a determination unit configured to determine a pressing strength to be actually applied to the fold line portion based on the first pressing strength acquired the first acquisition unit and the second pressing strength acquired by the second acquisition unit before the press unit flattens the fold line portion.