A post-processing apparatus to perform a predetermined post-process after an image formation process for forming an image, the post-processing apparatus comprising: an ejection tray configured to support a sheet stack ejected after the post-process; and a curl prevention portion including a first tray cursor and a second tray cursor respectively having distal ends configured to abut an upper surface of the sheet stack on the ejection tray to prevent the sheet stack from curling, and an angle changer configured to move the first and second tray cursors so that an inclination angle of the first and second tray cursors are changed with respect to the ejection tray , wherein the second tray cursor is distant from the first tray cursor in a first lateral direction that is orthogonal to an ejection direction of the sheet stack ejected onto the ejection tray , wherein the distal ends are distant from a predetermined rotation axis (RAX) downstream in the ejection direction, wherein the angle changer is configured to set the inclination angle to a first value when a number of sheets forming the sheet stack is a first number, and wherein the angle changer is configured to rotate the first and second tray cursors around the rotation axis (RAX) by setting the inclination angle to a second value different from the first value so as to make the distal ends depart upward from the ejection tray when the number of the sheets increases from the first number to a second number, characterized in that the curl prevention portion includes a position changer configured to move at least one of the first and second tray cursors in the first lateral direction or a second lateral direction opposite to the first lateral direction. an ejection tray configured to support a sheet stack ejected after the post-process; and a curl prevention portion including a first tray cursor and a second tray cursor respectively having distal ends configured to abut an upper surface of the sheet stack on the ejection tray to prevent the sheet stack from curling, and an angle changer configured to move the first and second tray cursors so that an inclination angle of the first and second tray cursors are changed with respect to the ejection tray , wherein the second tray cursor is distant from the first tray cursor in a first lateral direction that is orthogonal to an ejection direction of the sheet stack ejected onto the ejection tray , wherein the distal ends are distant from a predetermined rotation axis (RAX) downstream in the ejection direction, wherein the angle changer is configured to set the inclination angle to a first value when a number of sheets forming the sheet stack is a first number, and wherein the angle changer is configured to rotate the first and second tray cursors around the rotation axis (RAX) by setting the inclination angle to a second value different from the first value so as to make the distal ends depart upward from the ejection tray when the number of the sheets increases from the first number to a second number, characterized in that the curl prevention portion includes a position changer configured to move at least one of the first and second tray cursors in the first lateral direction or a second lateral direction opposite to the first lateral direction. <EOS>