The invention provides a sheet storing apparatus, a post-processing apparatus and an image forming system having the same. In an apparatus structure having a sheet discharge mode to store sheets at a stack tray from a sheet discharging path and a sheet discharge mode to store binding-processed sheet bundles with collating and stacking, sheet holding means is arranged above a sheet placement face of the stack tray as a sheet pressurization force being variable in two steps, and then, the pressurization force of the sheet holding means is adjusted in two steps in accordance with the sheet discharge mode for storing sheets at the stack tray.