To protect a loaded bundle after detecting full loading when a means for pressing a folded sheet bundle by a roller and a means for detecting the full loading on a loading tray are installed.

This sheet treatment device comprises a pair of folding rollers 826 folding a sheet bundle, a press unit 1200 pressing the pleat of the sheet bundle, a paper discharge roller 827 discharging the paper bundle, and a loading tray 832 loaded with the discharged sheet bundle. The sheet treatment device further comprises a full load detection sensor 1100 detecting the full loading of the sheet bundle on the loading tray 832. When the full loading is detected by the full load detection sensor 1100, press operation by the press unit 1200 is prohibited.