To provide a sheet processing device capable of buffering a specific number of sheets on a sheet transfer passage, and capable of quickly discharging the buffered sheets to achieve improvement in processing capability.

This sheet processing device comprises: a sheet transfer passage; and a sheet buffer device 5 including a sheet entrance means, a sheet discharging means, a driving means for driving the sheet entrance means and the sheet discharging means, and a sheet guide means for guiding the sheet so that the sheet passing through the sheet entrance means is stacked between the sheet entrance means and the sheet discharging means, wherein the sheet guide means is configured so that each leading edge of the sheet passes through the sheet discharging means while being stacked in a step-like form. A paddle 11 is arranged above the sheet guide means so that the transfer of the sheet is smoothly performed. The sheet entrance means includes a sheet entrance roller. The sheet discharging means includes a sheet discharging roller. The paddle 11, the sheet entrance roller, and the sheet discharging roller are simultaneously rotated by the driving of a drive motor as the driving means. The sheet processing device further includes a control device for increasing the rotation speed of the sheet discharging roller relative to the sheet entrance roller.