To provide a sheet post-processing device that can reliably align sheets and load the sheets on a stacker tray, that can perform various sheet post-processing functions at high speed and that has a compact structure.

Jogger drive means is controlled based on detection information from a sheet detection sensor so that the jogger repeats a motion of reciprocating within a predetermined distance from a standby position of the jogger toward a sheet width direction, and then a set number of sheets are aligned and loaded on a base tray 112 as a sheet bundle. After the set number of sheets are aligned and loaded on the base tray 112, gripper drive means are controlled so that a gripper 106 grips a rear end of the sheet bundle to push it, and then the sheet bundle on the base tray 112 is discharged to a stacker tray 103.