A sheet ejection device includes a sheet tray on which ejected sheets are stacked, a pair of side fences for restricting positions of the ejected sheets along a lateral direction perpendicular to an ejection direction, and at least one rib is formed vertically on each inner side of the side fences. The ribs are configured to contact with side edges of each of the ejected sheets while the each of the ejected sheets falls down to the sheet tray to align the ejected sheet along the lateral direction. The ejected sheets are aligned adequately by the ribs. According to the sheet ejection device, superior sheet ejection performance can be brought without affected by its environment.