In an ejected sheet stacking device for use in an image forming apparatus provided with a sheet ejection part formed in a hollow place formed so as to open to an outer housing of the image forming apparatus and with a sheet storing mechanism for storing sheets ejected from a sheet ejection opening of the image forming apparatus, the ejected sheet stacking device comprising: a plurality of sheet storing trays arranged in a multi-stage way in a vertical direction; an ejecting and conveying mechanism for conveying a sheet ejected from the sheet ejection opening of the image forming apparatus to one of the sheet storing trays; and a conveyance path switching mechanism for switching a bifurcating conveyance path which bifurcates the sheet ejected from the sheet ejection opening to lead to each of the sheet storing trays. The ejected sheet stacking device is mounted to the image forming apparatus in place of the sheet storing mechanism.