Sheet stacking apparatus and image forming apparatus

A sheet stacking apparatus comprising:

• a stacking portion on which a sheet is stacked;
• a conveyance member configured to convey the sheet toward the stacking portion;
• an abutment portion against which a leading edge of the sheet in a sheet conveyance direction is abutted;
• a belt member having elasticity and configured to move the sheet conveyed to the stacking portion, toward the abutment portion in the sheet conveyance direction;
• a nipping portion configured to nip the belt member;
• a lifting/lowering unit configured to lift and lower the nipping portion between a first position and a second position, the first position being a position for causing the belt member to contact the sheet stacked on the stacking portion, the second position being a position above the first position;
• and
• a controller configured to control the lifting/lowering unit such that, in a case where the belt member moves a current sheet and then moves a next sheet to be stacked on the stacking portion after the current sheet is stacked, (i) the nipping portion is positioned at the first position after the belt member contacts the current sheet and before the leading edge of the current sheet abuts against the abutment portion, and (ii) the nipping portion is positioned at the second position after the leading edge of the current sheet abuts against the abutment portion and before the belt member contacts the next sheet.