Sheet stacking apparatus, sheet processing apparatus and image forming apparatus

A sheet processing apparatus or an image forming apparatus has a discharging member which discharges sheets; a stacking member which stacks the discharged sheets; a sheet positioning member, located on the downstream side of the stacking member in the discharging direction, which receives ends of the discharged sheets in a discharging direction of the sheet discharged by the discharging member; and a pressure member having a pressure surface which presses the sheet received by the sheet positioning member toward a stacking surface of the stacking member; wherein at first a downstream side in the discharging direction of the pressing surface presses the discharged sheets toward the stacking surface of the stacking member, and subsequently an upstream side in the discharging direction of the pressing surface presses the discharged sheets toward the stacking surface sequentially.